Hey guys Ive installed GrapheneOS for like 5 Google Pixel a8. Every single one was ok only for one Phone.
This one is so frustrating It has unstable celluar signal I've updated everything and I get 4G for like 3s and then no signal.
I've tried 2G, 4G, Allow calling with data, Wifi, restart data and internet, etc. even reinstalling the whole OS more then twice.
I dont understand the logs and I have to do it cuz of work.
Can you help me?

The first thing i would do in that case is testing if the same behavior occurs with stock os. Maybe u did, but didn't mention it in your post.

This might very well be a (hardware) issue with the radio/modem.