Since currently (as of writing this post) there’s no stock firmware available for AT&T Galaxy S5, you can use a SafeStrap Recovery backup to restore your AT&T Galaxy S5 SM-G900A. Now, this is the Android 4.4.2 ND3 firmware, it will only work if you have not upgraded your bootloader past ND3 most likely.
Usually I don’t post this kind of stuff since stock firmwares eventually come out but since I was helping someone restore their AT&T Galaxy S5, I figured why not.
Basically, this will restore your AT&T Galaxy S5 to completely 100% stock Android 4.4.2 ND3 firmware. You will need SafeStrap already installed to install this stock backup. This is for users who have not backed up their stock firmware or ended up in a bootloop with custom ROM.
This backup only includes system and data, does not include bootloaders/modems/etc…
You just need to place the backup folder into TWRP backups directory but if you have trouble, please refer to the video tutorial above, thanks!