I am going through something very similar. My entire family is on the same T-Mobile plan, and on recent iPhones - however, my wife's phone is the only one where RCS fails to work over Wi-Fi (only works over cellular). I've reset her network settings completely, no dice. T-Mobile support is worthless on this and basically just offered to recreate her eSim (didn't work). Apple said I need to talk to T-Mobile, not them. When she's off Wi-Fi, it seems to work. I honestly have no idea what could be broken here.