This sounds a lot like the recent SIM disconnect problems that many people are describing (here). Try disabling setting location mode to battery saving or disable location services to see whether that avoids the problem whilst a fix is being worked on:
https://forum.fairphone.com/t/network-disconnects-gps-problems-and-re-entering-sim-possible-workarounds/20642?u=johannes&source_topic_id=20704
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