Currently the only option for a texting app is Samsung messages. This does not send MMS messages or group messages on wifi only. While that is bad, I of course could use an alternate messaging app (not a solution). THE REAL ISSUE IS NO MESSAGING APPS WORK IN SAMSUNG DEX MODE. AT ALL. So please tell me your plans to address both issues. I know you did this to make us use more mobile data. I get it and it's a bad move.
I fixed this issue by enabling phone permissions for the carrier login app and wifi calling app then making sure wifi calling was enabled. I'm pretty strict on permissions so my mms stopped going without these apps having phone permissions but it is needed for sending/receiving mms messages via wifi. This resolved my issue and now I am able to send mms with wifi in my Samsung messages app with all recent updates applied and I'm in DeX mode. Xfinity mobile is not trying to jack your bill. MMS messages are included with your free calling/text messages service.