I haven't seen separate APNs for RCS here†. Since the iOS support, what "offered by the carrier" means in most the world is only (de)registration through IMS service entitlement. Unlike the Google Jibe / Messages pairing done OTT using an SMS token to check your phone number, which is still the case in a lot of countries. Once the registration established, it's plain data traffic to Google servers.

† Here = "global" RCS, de facto controlled by Google. I haven't checked carrier settings for RCS islands such as the deployment in China or Korea.