How to set up Straight Talk MMS & Data with Nokia Network Setup app on Lumia 900
Setting up the Straight Talk is relatively easy now that the Nokia Network Setup App is available again from the app store. If you haven't downloaded it, go to the app store and search 'Nokia Network setup' and install. Here are the instructions for setting up the Straight Talk APN for both Data and MMS using the Nokia Network Setup application. Be aware that this application was pulled from the App market for a period of time and it's entirely possible that they may do that again at some point. As it is, the app is available as of this writing.
1. Select the Network Setup app from the main menu.
2. Once the App launches, you will see an Operator dropdown. This may or may not have more than 1 selection in your area. If you are in a 3g only area, then use the AT&T 3G option. If you are in a 4G area, use the AT&T - LTE1-Lumia 900 selection. That is the one that worked for me. You can come back and use the AT&T - LT2 - Lumia 900 if the first one does not work.
3. Tap the + icon to enter the setup screen and create a new connection. The connection you create will also show up in the Operator box.
4. Enter a Connection Name on the setup screen. This can be anything.
5. Tap the "Make this my current APN". It must be checked or your settings will not take effect.
5. Next, under Internet APN enter att.mvno for the access point name. This is all you will need for this section.
6. Scroll to the MMS section. Here you will need to enter the entries below:
MMS access point name: att.mvno
Proxy address: 188.8.131.52
Proxy port: 80
MMSC address: http://mmsc.cingular.com
7. The save icon will enable itself once you have entered all the entries. Be sure not to miss the MMSC address which is hidden by the keyboard. If you don't enter it, the save icon will be disabled immediately after you enter the proxy port.
8. Delete the existing APN settings If you have them setup in the standard cellular menu.
9. Once saved, you can test the data by launching the browser. Test MMS by sending a picture message.
10. If it does not work, repeat the steps using another Operator (step 2).
That's it, your Nokia Lumia 900 is now enabled for MMS and will work just like a contract Lumia.