Hi together!
So meanwhile I am testing the third different provider, just to be sure it can not be an coverage problem… but it is always the same problem. The signal-strength goes up and down in seconds, I can not send or receive calls.
Now I am using an old Samsung phone with the same SIM - at the same place, of course - and it works great. No problems with the signal, and I can phone. So I guess it is an problem with the phone, not the network operator / coverage.