I just chime in here without any great knowledge about these topics, and with my input probably not being the solution to the particular issues discussed here. But for somebody who encounter MMS problems it may at some time be useful to know about the following matters that previously has prevented MMS from working on my SFOS phone:
- In the SMS app settings there is a setting for Maximum message size. Setting this to extra large once solved my problems. (Can be carrier specific issues.)
- My phone had a twin SIM card that per definition from the carrier would not receive MMS. (also likely carrier specific.)