Glad you have got it sorted.
Just scanned through the posts on the forum dedicated to the Note 20 Ultra and cannot see anything similar. Therefore, I am inclined to think that your issue is carrier related. I have no issue with RCS on EE with my Galaxy S20 Ultra 5G.
For the Note 20 and Note 20 Ultra forum, see... https://eu.community.samsung.com/t5/galaxy-note20-note20-ultra-5g/bd-p/galaxy-note20-uk
If you turn it off does it reconnect though?
It seems people can't reconnect it. It's more like to be a Google than Carrier issue, both Google Messages and Samsung Messages are connecting to Google's RCS Servers.
Just turned off RCS and then waited and turned it back on again and it took 12-15 seconds to reconnect.
Just a thought, try Google Messages from the Play Store, which the Samsung Message app is based on.
1. re-install Google Messages app
2. check permissions / Settings → apps → Google Messages → Permissions
3. check the service with network provider if enabled on their end
Google problem :).
You're registering your number on Google's RCS server either using Google Messages and I'm pretty sure Samsung Messages. I'm not 100% sure on Samsung Messages as my brother on Sky can't get RCS in Samsung Messages (they don't apparently support it) but there's also this:
Either way it breaks because your number is registered with the service and for reasons no one seems to know can't be unregistered at this point of breaking.
I suspect Google would have to fix it but Google being so big don't offer direct support I'm aware of.