Downgrading from Android 5.1.1 to Android 4.1

I have Pac Rom Android 5.1.1 installed on my rooted sg3 and I need to do a carrier unlock. I contacted ATT to get the unlock code and the code they provided didn't work. Now I want to downgrade to Android 4.1 and try the UMTS menu trick as described here http://forum.xda-developers.com/show....php?t=2176719

I have tried Odin many times (with many versions, different computers, USB cables, etc) and every time Odin fails to flash Android 4.1. with a Fail AUTH error. I have used Odin successfully in the past many times with this phone but am unable to get it to work this time around.

Does anyone know how to downgrade the sg3 from 5.1.1 to 4.1? Or even better does anyone how to carrier unlock the sg3 when it has 5.1.1 installed on it?

Thanks!