Hi Stefanie2 - and a belated welcome to the forum!
May I re-affirm the suggestions made by @Martin_Anderseck in the second post of this thread. These immediately occurred to me too, in particular internal contacts: all the symptoms you have mentioned seem to be compatible with a potential internal contact / RF problem that might be alleviated by cleaning with isopropanol.
If you’re not confident with the idea of taking your phone to bits, I’m sure you’ll find help through a local Fairphone Angel.
Good luck! 