I have two FP2 and a AldiTalk/MedionMobile (O2/Telefonica) sim. In both FP2 I don’t get a 4G connection, though the sim works with FP3 and Galaxy S5. I switched Wifi off and did a reboot to the phone. The download speed with 2G is about 20KB/s. The OS is a recent LineageOS.
I got a SMS from the network provider that I should enter a special PIN to the sim to save the configuration, but the PIN didn’t work at all.
How old is your SIM card?
Did you check the APN of the provider?
Perhaps remove the APN, reboot the phone and enter the APN as new.
Is the SIM set to 4G in the settings?
Yes, 4G (recommended (translated from german))
I don’t know, It has 1.8V which means, it cannot be too old. It works in a FP3 and a S5.
Another SIM card without any details about its voltage from the same provider shows the same behaviour. We have no SIM card from another provider at home.
It is the same as in the Galaxy S5. It seems to be OK.
The APN was not allowed to be empty. I left an “i”. After an reboot it is fully configured again.
And you use only 1 SIM? Did you try the other Slot? Did it ever work? Did you test at different locations? Mine always worked with O2 so I doubt its a general provider issue.
One general comment: the fact the SIM is working in another does not mean its not faulty. Did you contact your provider, maybe they are willing to send a new one?
The FP2 and FP3 use a different SIM size, could be a problem as well and it was always easy to break the Pins in the FP2 slots when using an Adapter…so be careful
Did this change all of a sudden, was there some event that might have a bearing? OS update? I guess we can discount a dropped phone or dampness ingress since this is happening with both phones.
Did you contact the network provider to enquire as to how this PIN is to be used?
As yvmuell said, test in different locations.
I got the first FP2 for paying the postage and the other from @nudge to repair the first one, since I thought the mobile data problem and the reconnects of the wifi would be a general problem in the radio part.
Thanks, that is an important detail.
No, I didn’t think of a SIM card problem till now. Maybe I ask a neighbour for a SIM card.
I didn’t know it before.
If you cant find a solution and could wait or there is no meetup before: I should be in Hamburg on 05.+06.06 (maybe also before) and could bring my FP2 for a short test…
That is very kind. I make a note to my calendar and would write to you again at the end of may in case I didn’t solve it.
I will try that in the next days.
Well if your SIM fits the FP3 it’s a Nano SIM (4FF), whereas the FP2 takes a Micro SIM (3FF). The FP2 slot being larger, of course the SIM can be inserted but likely improperly placed for the contacts to align correctly. As stated above, an adapter can be used but that’s not advisable.
I would recommend you obtain a new SIM of micro SIM size.
I didn’t know, that the it was always easy to break the pins. 
This could always be the point: Use the first, left slot, if it is open, the right one won’t work for LTE. I had the same problem using 2 SIM cards. Both O2/Telefonica, still working today. Only the left one has LTE.
But again: Not everywhere.
Best
Peace
In your case its rather, because you use 2 SIM cards. only one can be set to 4G the other one must be set to 2G. Both should work with 4G in theory.
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