Galaxy Tab S 10.5 SM-T807a stuck at "Unauthorized system software" lock screen.

[AC Question]

Recently I purchased a used Galaxy Tab S 10.5 LTE [SM-T807a], and set it up for use as a WiFi tablet. I have a phone with a hotspot, so that part isn't really necessary. I had rooted it with KingRoot, and was attempting to shift it over to ChainFire's SuperSU program. As this had not been successful, I restored KingRoot, and as I had root reestablished, I tried using FlashFire to flash TWRP for the TabS 10.5 LTE [chagalllte]. This reported success, and the system rebooted; upon reboot, I got the following error:

System Software Not Authorized by AT&T has been found on your tablet. Please turn off your tablet and go to the nearest AT&T store for help.

I have made multiple attempts to recover through Odin 3; it has failed on every attempt. Hoping someone can help; if naught else, might have the correct ROM for this tablet. TIA.

 

[OS2Tech]

Turns out, I may have an answer to my own issue. After further pursuit online, I found the following thread on XDA-Developers forum, in re: this particular unit:

https://forum.xda-developers.com/galaxy-tab-s/general/stock-firmware-sm-t807a-t3492330

I'll be testing it tomorrow morning, but it may be the solution; it also has the Marshmallow ROM for the AT&T version of the tablet.