Cannot send or receive mms messages iphone 3gs

I have an iPhone 3GS running iOS 6.1.6 the phone is locked to AT&T. I use a Straight Talk SIM and have, voice, data, and Text but cannot send or receive MMS. I have gone through the APN update for Straight talk and even talked to spoke with a tech support rep. He said that he adjusted the APN settings for my SIM card and still not able to send.