Same problem here. An “x” appears on the signal strength of the 2 networks of my 2 sims after the update to Android 11 a couple of days ago. So I have no mobile data nor calls.
All settings seem fine (I haven’t changed anything and before it worked), but I will now go through all options mentioned in this thread, although I don’t see any clear solution reported yet.
Seems the same problem as reported by many other users here:
Hi and welcome to the forum.
The x indicates the mobile data it turned off rather than is failing to work or negotiate etc.
Updating can upset your personal settings so hopefully it is a simple reset that is required.
I note you have found a more suitable topic
All the best
Hi StepYourMind and welcome to the community and the forum.
I’m assuming that this problem is recent. Can it be linked to any specific event such as OS update?
Would you be able to test the SIM in a different phone?
Official instructions from KPN for configuring FP3 are given here:
I would suggest that you take a look, and if necessary create a new APN as indicated.
If the problem is hardware-related, it might be a good idea to get in touch with a local #fairphoneangel who might be able to lend you parts for testing.
Keep us posted 
Okay, so I let this rest for a while, because frankly I was just rather annoyed with it. But, I tried your suggestions:
I’m going to check the other thread to see if anyone’s found a solution but right now I’m thinking I might try to find out how to go back to Android 10.
Does anyone know if Fairphone is going to come with a patch to Android 11 anytime soon that might solve the problem?
May be more satisfying for you to contact support at fairphone to see if they are even aware of such a problem.
I don’t think that’s the case, since the topic you point to, while post-A11 like this one, is about mobile data and IP issues. Android 11 brought lots of temporary problems of different sorts, but those related purely to mobile data have mostly been fixed through adjustment or reinitialisation of APN settings, or taking the phone down to 2G and back up to 4G. It’s unwise to lump things together too hastily.
The problem we’re dealing with in the present topic involves loss of basic telephony (and not surprisingly, but as I see it only as a consequence, loss of data too).
I’m rather inclined to think that these call-related issues are coming from the combination of FP3 + A11 + specific carriers.
In view of that, it may be up to the carrier, or FP and the carrier, to fix the problem. It’s certainly important to make sure that both FP Support and the carrier are aware of each individual case.
Just an update. I tried a factory reset after backing up my photos etc., but it didn’t help. Still no connection to the mobile network. I’ve mailed FP Support to let them know about the issue.
That’s good, and I would do the same with your network operator. This may be a problem that needs action at both ends!
I placed my work-SIM into the phone (different operator: simyo instead of KPN) and it had the same problem. So I’m inclined to think it’s the phone somehow. But I’ll contact KPN just to be sure. Thanks!
Hello again,
Since my last post, I have indeed tried a network reset to no avail.
I have contacted my provider - Vodafone - who have looked at the network twice, and couldn’t find anything wrong.
I read a factory reset isn’t helping either, so I’m not inclined to try that.
In the meantime, I noticed that; calls just outside my home are now ok again.
My 4G or 4G+ reception is bad in; certain restaurants, shops, trains, trams, busses and both phone reception and 4G/4G+ internet reception are bad is the battery goes below 30%.
I also checked the phone itself, as I did drop it, and noticed there’s a crack in the bottom, in the plastic outside next to the USB-C port, don’t know if that could be part of the issues I experience. And, to add; while I am not sure from which exact date the connection problems started again, but I know I dropped my FP3 later than that.
I have now contacted FP support.
Hopefully something can be done.
EDIT 1: it’s two weeks later and unfortunately I have not heard back from support yet 
EDIT 2: I should have waited a few more hours as I just received an email from support. Hurray! 
I’ve been in touch with Fairphone Customer Support. They say they’re investigating the root cause of the issue, but currently don’t have a solution.
They’ll keep me updated on any developments.
Has anyone else had any luck with workarounds or solutions to getting back a mobile network connection?
The problem seems to your set up, not a general one, so I doubt anyone has had any luck as they may not have the problem. Even the OP has a slightly different problem.
So to confirm, you have no reception via network or Wi-Fi with two SIM cards, one that in new, and they both work in an FP4 neither work in the FP3
I have no reception via network for calls and mobile data in my FP3, since (probably) the OS update. Both calling and data worked fine before May. I have 1 SIM in my FP3, in slot 1. Switching it to slot 2 makes no difference.
Switching my SIM to a FP4 immediately solves the problem and the SIM connects to my carrier’s network just fine. Putting a different SIM in my FP3 encounters the same problem: it just doesn’t see the mobile network, whereas in a different phone, it connects just fine. So yes, the problem seems to be in the FP3 somewhere.
Doing a factory reset has made zero difference, so it’s not a problem with “my” setup insofar as that I’ve done nothing out of the ordinary with my phone (as far as I can tell?). It’s a mass-produced phone, the carrier is the biggest one in my country, Fairphone OS built on Android 11 has seen a global rollout. It should work, but somehow, it doesn’t.
I do have reception via WiFi on my FP3, though it has been more unstable since May (it frequently loses connection and then has to reconnect). This is the only way I can still make calls or receive messages, but this doesn’t really help me when I’m on the road.
You might have a look at the APN settings.
Some info here might be of interest
Update:
Have been in touch with Fairphone Customer Support.
One suggestion was to get a new SIM card, as mine was a bit more than 3 years old.
Went to a Vodafone store to have it replaced, after which I had no mobile data (4G or 4G+) at all. They tried and activated a second SIM card, and it didn’t work either.
When putting my old SIM card back in, I had mobile data (4G/4G+) again.
EDIT 1: So, it’s not the SIM card that’s causing my reception problems.
EDIT 2: For clarity’s sake; the original problem - sometimes not being able to make or receive calls in and around my house, and in certain other buildings and/or when the battery is less charged (> 30%) - hasn’t been solved yet
Interesting. I didn’t know it’s possible to re-use an old SIM once a fresh one has been activated for the same subscriber number? Just wondering whether those “new” SIMs were properly activated and recognized by the network.
According to two people working in the Vodafone store; both SIM cards were activated properly.
It would be an amazing coincidence if this were to happen twice in a row.
Today, the same thing happened, and could not make or receive calls in my house. I had to walk outside to have an important phone call.
And to be honest, I’ve just about had it with (this) Fairphone. The suggestions and support I receive heavily point in every direction but two; Android and/or the Fairphone itself.
I’ve had these issue with the FP2, and minus a period where it at least seemed to work fine, now with the FP3.
It’s not like I demand the phone to calculate infinity, while streaming every Marvel show in order, curating playlists by random themes, while on every social media all at once; I want to make and receive phone calls, the most basic function of a phone and it can’t properly do that.
UPDATE/CONCLUSION.
After several emails and the summer holidays, I finally send in my FP3 back for possible repairs. I have since heard back from Cordon, and it turns out…the entire thing was thoroughly oxidised. Well. F**k,
It took me awhile, but I suddenly remembered that there once was fluid behind the lens of the back camera and in my desire to repair it myself (because; Fairphone), I did the rice-in-the-fridge trick. The fluid was gone, but it turns out it wasn’t the solution after all. If only I had send it in for repairs right then, it’d still be under the warranty. Oh, hindsight…
The quote on repairs was € 429,-, so I decided to have it recycled.
If I was going to spend that much money, I might as well add a bit more and purchase a new model.
I very much hope my reception issues will be over when the FP4 arrives.
This took a bit too long; from experiencing problems in the beginning of this year, to finally contacting Fairphone in July, to now getting a new one.
That said, and apart from having my emails not properly read once or twice, I found the Fairphone Support team professional, supportive and overall very pleasant to deal with.
Anyway, anyone that can use a FP3 protective case?
If the moisture indicators in the phone would have shown that there was exposure to fluid inside (there are small white areas that irreversibly turn pink or red when wet to prove exposure), then you would not have been under warranty anymore.
Try the Market category 
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If the moisture indicators in the phone would have shown that there was exposure to fluid inside (there are small white areas that irreversibly turn pink or red when wet to prove exposure), then you would not have been under warranty anymore.
Don’t know about any moisture indicators. Where are the located?
And why wouldn’t it be under warranty anymore? I imagine it depends; if one drops a phone in a lake it’s a different situation to being exposed to rain.
Try the Market category
Ok. Will do.
Why the wink though?