Nokia Lumia 900 on Straight Talk

[WPCentral Question]

I recently purchased a factory unlocked Nokia Lumia 900 with windows 7.8. I bought an AT&T sim from straight talk and activated it I put in the APN settings I was told to using the Network Settings App I can use internet, regular texting and calling but I cannot send or receive multimedia messages and I cannot use group messaging. I have looked at so many different settings and APNs but nothing works. Please help!