I've had a problem with receiving RCS messages from other TMobile customers who have chat enabled are are not using Google Messaging. I'm using the native Samsung Messaging App with a Galaxy S20.
The only way I can receive messages from everyone is if I switch to the Google messaging app which I don't want to do because I have an Active 2 watch.
I've done it all. Cleared cache, reset settings, made sure delivery and read is on for everything. Factory reset my phone, customer service with TMobile, customer service with Samsung.
I don't receive messages from people who have a blue dot next to their contact image. They can receive my messages.
I've narrowed it down to TMobile customers.
Just thought I'd check in to see if you got this working? I just set up my S20+ (T-mobile) and have the same issue. RCS isn't working with samsungs default messenger although chat settings are enabled. For now I can use Google messages since I have a wear OS watch, but I would like to get a Galaxy watch. I won't purchase one though if I can't RCS on samsung message app :-/
I'm actually testing it again now.
First day with a contact I didn't receive messages from. The first few have come through RCS. We will see.
Side note, you can receive Google messaging notifications on the Samsung watch, and can reply; however, you can't initiate a text unless you're using Samsung messaging.