I don't think it's from your end.

On Sailfish using T-mobile USA, the "fast.tmobile.com" APN does not work for MMS, but the "epc.tmobile.com" APN does.

It sounds like MetroPCS is using T-mobile's "fast" APN. If MetroPCS has an equivalent APN to T-mobile's "epc" APN, then it might work if you can somehow switch to that.