Originally Posted by yoonus
Do you have the SM-900A ATT model? Or are you using T-mobile phone with ATT network/SIM?
The ATTS5 Model can be sim unlocked but has a locked bootloader and as of now can't be rooted so you can't flash Roms. The T-Mobile S5 can be sim unlocked, rooted and can flash tmobile or international 900F roms. You will have to provide a APN to get LTE.