I have an ATT GS4 I337 which I have been using it on Tmobile Network for a while. To convert it into a fully functional T Mobile phone, I have went through many threads and melted down the steps to what you will see here. It works for me and I am using it as my daily device.
On different threads I saw many people were asking about the same issues that I encountered and some could not figure it out how to overcome them.
Finally I decided to put the steps I went through along with links to what I have used to make the process noob friendly.
Note: I am not the main developer of what is being presented here. What I have here is combinations of what I have seen on different threads.
Disclaimer: You are the one who are deciding to use what I am presenting here, All the risks are on you.
NOTE1: my phone has a locked bootloader so I am only explaining what needs to be done for locked bootloaders. If you are not familiar with this concept, make some research and make sure if you want to proceed.
NOTE2: This has nothing to do with sim-unlocking the phone. If your phone does not accept tmobile sim, first sim-unlock your phone (I am not Google!) then come here and go through the steps.
I assume that your ATT i337 has been soft bricked and therefore I will go through all the steps from bringing it back to life until converting it to a M919 equivalent.
2-Bring the phone back to life (fresh start)
3- Update to Latest ATT ROM (NI1)
4- Root NI1 Rom
5- Installing SafeStrap
6-Converting it to Tmobile Phone
7-[Optional] Installing missing Tmobile/Samsung apps/widgets <--> and UNDO it!
8-[Only IF] you do not want to be on NI1