Which carrier? Typically customer service reps are low-level initial support folks who follow scripts. You’d probably need to ask to be escalated to someone technical to find out what’s going on.
There are a couple of reasons why a carrier might have incomplete support for RCS. One is that, historically, carriers were not required to support RCS at all. So, especially for carriers that still use 4G, or which are piggybacked on other carriers’ systems, they may either have limited capacity, or may even be using one of the older versions, such as Universal Profile 1.0, that may not be able to be provisioned for devices that require later UP versions.
Fortunately, the 5G specifications do require carriers to support RCS, since RCS is intended to replace SMS. This drives wider adoption, and hopefully, all carriers will be able to support UP 3.0 later this year.
RCS requires the carrier to host their own RCS message relay servers. This has nothing to do with 5G, 4G etc.
RCS works over any connectivity method, meaning it works over WiFi with no cellular connectivity.
The history of RCS rollout:
-Originally, RCS required carriers to provide their own message servers for RCS support.
-Google got tired that carriers didn't do this, so Google now provides their own RCS servers (Jibe) for Android devices world wide to connect to, bypassing the carrier having to provide their own RCS servers.
-Google doesn't allow Apple devices to talk to its RCS servers directly without the carrier having an agreement in place. Once an agreement between the carrier and Google is established, the carrier defines the Jibe servers in the carrier profile config.
-Carriers have been very slow to do this, because it costs them money to host their own or negotiate a deal with Google to use theirs, so it's something they avoid doing as it costs them money to implement.
Not a single Australian carrier supports RCS, and they are all 5G networks.
Australian Android users can use RCS because their devices communicate directly to Google Jibe servers without the need for a carrier profile instructing the device to do so. This is why Android devices support RCS and iPhones don't.
Android devices have the Jibe servers defined by the messaging app. While iPhones rely on the carrier profile to define the servers.