I have an Unlocked HTC Desire 610 from AT&T that i use on MetroPCS (with T-mobile network APN) and it stopped sending/receiving MMS messages. I fixed it and here's how I did it in my APN Settings:
Name: MetroPCS
APN: fast.t-mobile.com
Proxy: Not set
Port: Not set
Username: Not set
Password: Not set
Server: Not Set
MMSC:
http://mms.msg.eng.t-mobile.com/mms/wapenc
MMS Proxy: Not set
MMS Port: 80
MMS Protocall: WAP 2.0
MCC: 310
MNC: 260
Authentication Type: Not set
APN Type: default,mms,supl,hipri
Bearer: Unspecified
The only thing i had changed was adding the MMSC of
http://mms.msg.eng.t-mobile.com/mms/wapenc and my APN Type was originally set to default,supl,hipri . Adding the mms to the APN type was what fixed it for me. Hope I can help any future inquiries of this question with this post.