What have you tried? Things should "just work" if you install ofono and MMSd. There are more details (urls, which forks are involved) at https://sr.ht/~anteater/mms-stack/

The basic debugging workflow is to run ofono, run MMSd, and try the scripts under the `test` directory in MMSd's source tree. Error messages logged from the script, MMSd, or ofono might be relevant.

MMSd (mms parsing, mostly) and ofono (dual-stack IPv4/IPv6 networking) need patching to make MMS work on T-Mobile; other providers might also need debugging or custom patches.