I have the S21 Ultra with AT&T. I'm running Android 12/One UI 4. I can't seem to get the Advanced Messaging to work. My husband also has a Samsung phone and the Advanced Messaging features work with others but not when Messaging me. Has anyone had similar issues?
Are you referring to "Chat" option?
The option is there, if your carrier supports it.
Check to see if you have Carrier Services installed
While there have been some users that say that Advanced Messaging (Chat, RCS) works with AT&T there are others who say it doesn't. My suggestion would be to call AT&T and ask for yourself.
Galaxy S21 Ultra 512g(VZW), Galaxy Book (original, VZW), Tab S4, Galaxy Watch3 (VZW)