I'm having trouble flashing switch to my Galaxy S4 i337 (AT&T varient). I am able to flash both roms. Touchwiz boots up perfectly. However, there is no WiFi and when I attempt to switch to the Stock rom. My phone restarts about 2 times and then a screen that says "Software that is not authorized by AT&T has been found on your phone" pops up. I have installed the Touchwiz AT&T stock rom and used TWRP to flash both roms, but they have not worked. Thanks in advance for help.