Hey guys,

I bought an ATT Samsung Galaxy S4 and unlocked it to use with my TMobile SIM card.
For some reason, I've been having lots of problems with sending/receiving picture messages. Sometimes it would work and go through right away, other times it would keep loading and eventually tell me that the message cannot send. Same with receiving them. Sometimes I would get the picture messages instantly, other times it wouldn't even let me download the picture.

What is wrong with my phone and how can I fix it?

My Mobile Data is turned on. My APN settings are as follows:
APN: epc.tmobile.com
MMSC:
MCC: 310
MNC: 260
(all other areas are: not set)

If you have any suggestions or advice, please help me out. I would really appreciate it! Customer service support was a waste of time.

On another note, I also can't make changes to my lock screen. Is it the fact that my phone is unlocked so I can't access and modify certain fields?