Setting up Data and MMS by Entering the Cellular Data Network Settings
You can set up both Data and MMS on your iPhone by entering the your APN Name and MMS configuration values into the Cellular Data Network option in your iPhone’s settings. You need to know your APN name in order to know which other MMS values to use.
This all sounds really simple but there is a problem. When you have an ATT network SIM in your iPhone, the Cellular Data Network settings are hidden. When your iPhone sees an ATT Network compatible SIM in your iPhone it automatically programs the Data APN and MMS values for you. And since it programmed them for you, it hides Cellular Data Network so you don’t go in there and mess it up. The problem is it programs in the AT&T values which are not the correct values for Straight Talk.
If you are using the Straight Talk T-Mobile network SIM in your iPhone the Cellular Data Network option is visible and it is easy to program. Ironically, your iPhone is more compatible with the AT&T network then the T-Mobile network. If you use the Straight Talk T-Mobile network compatible SIM it will be easy to set up data and MMS but most people will get much slower 2G data.
Fortunately you can use a hack called the SIM swap to work around the problem. You stick a T-Mobile SIM in your iPhone and verify you can enter the Cellular Data Network option in Settings. You then remove the T-Mobile SIM, put your Straight Talk AT&T network compatible SIM back in your iPhone. You race back to the Cellular Data Network before it disappears. Your iPhone will populate Cellular Data Network with the AT&T network values. You enter in the correct Straight Talk APN and MMS values for your account and save them.
I do not know how to setup MMS for the Straight Talk ATT network compatible SIM without a T-Mobile network SIM. The SIM does not need to be active. The SIM can be for the the T-Mobile network but not T-Mobile branded, for example the Straight Talk T-Mobile SIM will work. If you have an old SIM from some other network other than AT&T you can try that too.