One thing I would suggest with any troubleshooting is to restart all devices. Since you know 2 were on rogers, were those two able to see the activity? Could be communication issues between carriers also.
Hard to tell since this technology is still kinda new to the carriers and android. You'd have to reach out to the carriers also to see if they can determine if the connectivity is there.
yes, it's supposed to work across the carriers. But that depends I guess if they carriers want to play along with one another...i mean they fix the prices together, why not fix RCS together.