For your original post, your suggestion looks good to me. Just one thing to point out, you do need to manually turn on Mobile Data in Settings/Mobile, it’s not done automatically for to send/retrieve MMS.
One thing that has broken MMS for me in the past was my provider seemed to switch to IP6, which I think is not supported by the software.
You could try running this command to check you get IPv4 configuration? journalctl -b 0 | grep modem-broadband
(It goes through all the log messages since you booted the phone, and outputs those matching the string “modem-broadband”).