Pixel 8 on T-Mobile. I also had to use the Vuraze method of installing an older version of Messages to get connected. I tried the Aurora store version, but as others found, could not update to the Google Play version after that. But the APKMirror version did not have that problem.
I tried @SillySaurus's method first, several times. And I tried @t1c's suggestion to also delete storage for CarrierConfig2 and RcsService. But I would just get stuck at "setting up" every time.
But when I installed the old version of Messages suggested by @Vuraze and followed the suggested steps in @Vuraze's post, it connected instantly. As soon as I opened the app and selected to use my account it said welcome to RCS or something like that.
I really does seem like there is something about T-Mobile and T-Mobile based MVNOs that's different and require installing the old version of Messages to get RCS working. I don't know if this is something the Graphene devs can address or if it's a problem on T-Mobile's side.
Also, I did not disconnect from my VPN. That was never an issue for me in the past year when RCS was working fine. Although, like for others, at first I was connected but could not send messages and RCS connection state in the debug menu said "false" for Etouffee. That remained true after two reboots. Then I disconnected the VPN and rebooted again and after that Etouffee was "true" and everything started working.
It was not really scientific of me to disconnect the VPN and reboot a third time at once, so I don't know what fixed it. I should have checked after I disabled the VPN, but before I rebooted. Or rebooted a third time first and checked Etouffee, before disconnecting the VPN. Still, at least two other people have said it took three reboots to get things working. But that could be a coincidence.
Anyway, one problem I have now is that I was dropped from all previous RCS group chats. I got no flood of missing messages once I reconnected. How to I get added back to those RCS group chats?