I had the same problem recently and I'll tell you what worked for me on my Droid Ultra XT1080, because it's a little different than the answer given.
Preferred network type: LTE/CDMA
APN: fast.t-mobile.c*m (this won't let me post links)
MMSC: h*tp://mms.msg.eng.t-mobile.com/mms/wapenc
APN type...