I found these for you to try... When I was on T-Mo with an unlocked 1520.3 it just worked and I never tweaked anything.
Go to Settings > cellular + SIM > SIM settings > edit MMS APN
APN: epc.tmobile.com
User Name:
Password:
Authentication Type: <I can't edit this - leave as is>
WAP gateway:
WAP gateway port:
MMSC (URL):
http://mms.msg.eng.t-mobile.com/mms/wapenc
MMSC port:
Maximum MMS size:
IP type: <leave as is>
If I left a field blank, leave it blank in your phone...
Authentication Type - IDK... mine says "PAP" now, but I don't remember what it said when I had T-Mo? I wouldn't toggle this unless all else fails.
IP Type - it should match whatever you see under the same value in Settings > cellular + SIM > SIM settings > edit APN