My test running Ubuntu Touch on my Pixel 3a XL as my primary phone continues. I received an MMS for the first time today. Instead of displaying the MMS, the default Ubuntu Touch messaging app told me it could not be downloaded. I tried fiddling with the APN settings to no avail. Then I found the solution in a long UBports forum thread titled No MMS with Wifi ON. I read the thread and tried the recommended solution of turning wifi off and then downloading the message. This worked. However, I agree with a number of commenters in the thread that (1) MMS should ideally work with wifi on and (2) at a minimum, the error message should note that MMS does not work with WiFi on (unless your phone is receiving internet from another device via mobile hotspot). Of course, I cannot receive MMS on my Pixel 6a (running GrapheneOS) with WiFi calling on, so MMS may indeed be a complicated issue.