I have the same problem and your workaround saved my ass, don’t know what I would do otherwise. Thank you very much!
For me WIFI and microphone still work properly 24h later as well, with the workaround. Hoping for a quick fix still.
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Hi Lars, sorry for my late reply. It was (and actually am) very busy the last weeks, which is, why I couldn’t do any further research for this issue. I switched my eSIM to my business phone and therefore I’m fine for the time being.
What I found out in the meantime is, that, when I simply boot it via fastboot with boot.img it works perfectly fine. I had it running for the last couple weeks straight, of course without root due to the regular boot.img (Jeez, do ads stink 
)
I just made sure not to run out of battery power and I was fine. Last night though, I forgot the charge it and I had to redo the whole processes of directly booting boot.img via fastboot, because I’ve experienced the issue again elsewise 
Yes, my backup was written to the internal storage. Since I had it running (and now have it again running) I tried to pull my backup just via file transfer, but after quite some time it got awfully slow and could not be completed. Don’t know what that was all about.
I will take a deeper look into SwiftBU. Thx for the hint!
Hi CransNeighbour,
thank you for your very answer!
As you can see, I already figured out, that booting directly into boot.img via fastboot works surprisingly fine 
After reading your post here I tried to patch the boot.img which I used for booting into via magisk, just the way I would usually patch any boot.img. But when I tried to boot straight into the patched boot.img via fastboot I ran into the same problem as before: My phone would be stable after booting as long as it has power. But I can’t use it since it reboots like 15 seconds after entering the unlock code. BTW, I am almost certain that it’s not a problem with a SIM PIN, since I usually use an eSIM (without PIN) and right now don’t use a SIM at all.
BTW, I don’t experience any of your describes problems. After booting into regular boot.img my phone works, as described. perfectly fine. Wifi is not a problem at all. When I am not at home, I will simply open a personal hotspot on my business phone, where my eSIM is, and connect to it. Then I use it pretty much the way I would usually do. Well, besides being annoyed by the disadvantages of an unrooted phone
Have you had the chance for some more diggin into the problem by any chance? What makes you think that the issue could be resolved relatively easy? Would be great of course.
It is definitely not the SIM pin. And it’s most likely neither one.
EDIT due to some weird limitations: Has anyone by any chance tried already, if the new software update solves this issue here as well?
EDIT 2: The Update does not resolve the issue! But booting straight into boot.img as a workaround still works like charm.
Has anybody ever managed to bypass Widevine restrictions? To make it back to L1 again?
Does anyone know where I can find a boot image? I’m trying to root my phone and I’ve unlocked the boot loader, I’ve got Magisk, but even though I used it to try to install what I thought was the patched image, the phone is still unrooted.
Moved your post here
Maybe here you find what you need
else maybe hints above
Sorry for hijacking the thread.
I just updated my FP4 to LineageOS 21 and wanted to add root access via Magisk
However when I flash the boot (or just boot the patched boot image) I have no wifi anymore.
Now I read some posts that this seems common. Is this really how it’s meant to be? With LOS20 I had a patched boot image where I always had wifi and root (if needed). But now it seems I have to decide and either boot “normally” (without root) or with the patched boot image (and then without wifi).
But how am I supposed to make my backups to my server if I can’t have both?
— EDIT —
I found the solution on how I did it the last time. This post here describes it very well:
Step-by-step:
Patch the downloaded stock boot.img with Magisk
Transfer / adb pull that patched boot.img to your PC
Reboot to fastboot adb reboot bootloader and temporarily boot from the patched boot.img fastboot boot /path/to/patched_boot.img
Do not flash that boot.img!!!
Wait for the device to boot (took a bit longer than usual)
Open the Magisk app (press Cancel when asked to perform Additional Setup), choose direct install and reboot
You seem to have solved it already but in case you run into something similar in the future:
Wifi/Bluetooth/etc. problems are usually an indicator that you aren’t booting the right boot.img, did you patch the one of the LOS version you had installed? 
(Not that it matters really, if you can get to Magisk from a temporarily rooted system and run the proper install you’re good to go)
Oh and welcome to the community 