The solution is as follows (in effect this keeps the MMS connection active, which for some reason drops when using WiFi).

Android Settings > System (Advanced) > Developer options (turn ON) > (NETWORKING) set Mobile data always active = ON.

Note: If the Developer options cannot be found, you need to "unhide them" first, as follows.

Android Settings > About phone > Software Information > keep tapping "Build number" till it says developer mode on. Then locate (search) for Developer options in the settings.