Thank you for your responses. I will try to get the logs and paste them here.
@de0u , WiFi calling does work.
@Kottonballs , yes I did, I went through all the SIM settings.
The behavior of the phone is extremely strange. It looks like the problem is not with the hardware, but maybe with the operators I am using. Or the "reception towers" rejecting my device. After 24hrs of the update, I suddenly got reception (still being on the same spot). First just edge, about an hour later 4G. Went to sleep, put on Airplane mode and now I am again without reception.
In the chatrooms I got a suggestion to reapply the update via
adb. @GrapheneOS , is this a possible solution?