straight talk byop Lumia 635 at&t no mms

I bought the phone at&t go phone Nokia Lumia 635 and purchased the at&t byop sum card from straight talk, I have been able to get the internet at up everything works perfectly and is blazing fast at 4g lte but I cannot for the life of me get my mms set up so I can send picture messages, my wife wants to buy a new Lumia also but doesn't want to give up picture messaging to get one. Someone please help me remedy the situation I love my new phone.