Hi Abell, don’t worry, I had exactly the same problem and was so frustrated for the last three months but it came to an end after contacting the official Fairphone support. Here’s their second answer (after a first obvious solution I had already tried) :
"This is indeed a known issue, the antenna cable seems to be loose, if you are a bit handy
or know somebody that is handy, you can fix it yourself:
If not, we’ll have to swap your phone for a new one.
Let me know what you prefer."
I’m not particularly handy with electronics so I got my son to do it with the right tools (he is able to take a computer apart and put it back together). He couldn’t see anything loose but he took it apart, checked it, put it back together and since then I’m REALLY glad to say : IT WORKS PERFECTLY FINE AGAIN. So good luck to you !