mms on iphone 4s with straighttalk

I have a factory unlocked i-phone 4s and I am using a Straight Talk sim card. This has worked well for several months. Last week I started having trouble with my 4G connection. I called Apple and they walked me through re-setting the Network settings. The 4G connection worked great, except I could no longer send or receive mms messages. My data network tab is not on the phone. Apple said that it was my carrier but after speaking with Straight Talk I was told that the problem was with my phone. I took my phone to the Apple store and they ran diagnostics and could not find a problem. A friend with the same phone, except they have an AT&T sim card, put her sim card in my phone and once again, the mms messages did not work. Any suggestions????