In Phosh, the MMS server settings are store in Chatty rather than with the APN settings in the Network section of the System Settings app like in every other operating system.
And since Chatty only supports using one APN, if you switch APNs, MMS stops working. On normal OSs, you can set an MMS server for each APN separately. On Phosh, you can only do MMS on one APN.
mms works pretty well if you live in of those parts of the world which still uses mms (some regions rely heavily on them and others have disabled them)
