Have you check the forum for people having issues with the antenna? There was also something about GAPPS (Google Software) that can cause problems like this. Lets see if I find it again. Maybe it’s related. Else: Read the blog post about the latest support changes, go through their checklist … and if that doesn’t help … try to send it in.
Update, Links
Antenna, HW
Google Software causing issues (But this often involves a “Enter PIN” request you have not described)
https://forum.fairphone.com/t/fixed-google-bug-network-disconnects-gps-problems-and-re-entering-sim/20642