Have you considered using RCS? RCS is the intended replacement for SMS/MMS both of which date to the 1990’s.
RCS is supposed to work on any reasonably modern Android running current versions of Google Messages. Apple supports RCS on iPhones running iOS 18 or later.
Unlike SMS/MMS, RCS is engineered to work over Wi-Fi as well as cellular networks. USM support agents are correct in saying SMS/MMS aren’t designed to work over Wi-Fi though in some configurations they do. In my experience, when a support agent says something is unsupported it means while the something might possibly work they aren’t going to fix it if it doesn’t.
Up until now, I haven’t really had a need for RCS because everything was working reliably over Wi-Fi. From Republic to Boost to Mobi, I’ve always been able to get everything over Wi-Fi working just fine. That’s why I was surprised at them making such a definitive statement.
I had them switch my Pixel 4 to Warp (Verizon) and now MMS works perfectly over Wi-Fi. I also asked them about my wife’s phone and they were able to switch something on their end so now her phone is working over Wi-Fi as well on Dark Star (AT&T).
Thanks @cbwahlstrom, @davenc, and @rolandh for chiming in! I always appreciate the input people have on this forum.