Root [TMobile] Help please someone..

[slgdad]

I have a Galaxy s5 sm-g900t its a T-Mobile phone but I have at&t service.. I rooted with towlroot and Phil recovery installed. I did a backup of my stock ROM and firmware but after flashing a custom ROM I lost my backup somehow. So I flashed a stock firmware through Odin but I now I can't get any ota updates it gives me a message saying the operating system on your device had been modified in a unauthorized way.try downloading software updates using kies on your PC.. I tried that and nothing.. The firmware I flashed was 900tuvu1bnh5.. The only thing's I didn't do was a factory reset through recovery after the flash.. But after I flashed stock firmware everything was switched to T-Mobile! Not at&t like it was. I had to take it back to the store and have them switch it back to at&t I could make calls but no internet service at all.. The device status still says custom not original.. How can I fix this and if I do the factory reset through recovery. How do I get my network and AT&T working back?? Any help would be greatly appreciated..