First of all, it's now working flawlessly.
But i took the hard way, caused by another problem two days later with the system update installation, which got stuck for a day and wasn't finishing.
So i decided to factory reset the device, but it didn't helped at all. System update installation still stuck and the SIM not working properly.
The device was wiped, seed vault didn't worked for me, so i choosed to install the OS again with the latest updates over the bootloader. SIM in the phone.
After installation and rebooting the phone for the first time after installation, it instantly showed me my mobile carrier and no error occured anymore.
System update is now working like it should.
I don't know what caused these issues but for me it's solved.
Anyways i ordered a replacement SIM which i really don't need anymore :)
Im comeing from LOS and so far im really happy with GOS. You got more options to secure and personalize your device on your needs.
And thanks for all, i appreciate the support and the advice in this community, even if there is no solution in first place, i got the feeling at least somebody cares.