Understood. The provider is T-Mobile, and I don't have a contract.Beside the usual suspects logcat, dmesg and kmesg I don't know what else you could use. Also I don't know what to look for in them for this particular issue.
My best guess is that this has something to do with your service provider. It looks like they allow you a short window of unauthorized access to complete the registration process and if it doesn't finish you get locked out.
What provider do you use? Does your contract include VoLTE?
Like I said many times before I don't have VoLTE where I live so I cannot really provide support for this. The only solution I have was taken from PeterCxy when I copied his repo and I didn't dare to change it since then as I cannot test it all.
Since this is my daily driver, I rolled back to 2021-05 as I ran out of time to troubleshoot.