Google RCS and RCS the standard are not the same thing. Third party apps are not allowed to plug into Google RCS. This is why you won’t find any RCS open source alternative for Google Messages.
Google is just trying to promote their own walled garden under the guise of an open standard. If they were genuine about this they should allow you to just use any replacement, just like you can replace the stock SMS app on Android.
As far as I know Google doesn’t allow third party apps to plug into RCS.
This is why them bashing Apple for this particular issue always seemed hypocritical to me, they want this to be their own closed ecosystem, with Apple being the exception because they have enough clout to actually go it alone or even take users away from Android.
Ideally you’d have apps like Signal plugging into the same end-to-end encryption for interoperability, but Google won’t allow that because they just want people to use Google Messages for RCS, and nothing else.
Do the stores not have free WiFi? I’ve never been in a Walmart, but all bigger stores here in Europe have WiFi.