Fritz!Box supplying a 5 GHz Wi-Fi network by any chance?
In this case set a fixed channel and make sure it stays below channel 116 … https://forum.fairphone.com/t/my-fp3-doesnt-like-higher-5ghz-wifi-channels-especially-channel-116/54710/48?u=anotherelk
I would advise to open an issue in the GitLab and be prepared to supply log files, else it could be very hard for the developers to reproduce this in order to be able to fix it.