I have the same issue with my Note 9 and am looking for answers.
There is also a posting on the Xfinity forum regarding a user with a Samsung device that is having issues with MMS over WiFi.
https://forums.xfinity.com/t5/Xfinity-Mobile/Home-Wifi-not-allowing-MMS/m-p/3255330
Could it be related to what you're experiencing? There isn't a resolution over there yet. MMS works fine over WiFi with my Xfinity service using my Droid Turbo 2 and my iPad.
Start a new topic or ask questions in the open forum.
