I've had this phone for almost 3 months and have not been able to register for rcs through google messages. I've read the forums where people leave it off for days and then come back and turn it on. Didn't work for me, I've deleted google messages and reinstalled, didn't work for me. Restarted my phone, popped out sim card and restarted. Nothing works.
I don't even understand about registering a number for RCS. I have always used Google messages and even when it updated to include RCS, RCS was automatically enabled, I didn't have to do anything like register a number -- just use it as my messaging app as I have always done.
Try booting into maintenance mode and deleting your cache partition. See if that helps. You may have to take the nuclear option and factory reset and restore your backup if that doesn't work.
Did you turn off RCS on your previous phone, if not that could be the issue and you might have to wait a couple of weeks until it releases from that phone. Google use to have a page you can go on to deactivate it on an old phone but I don't have the link
