Do you have access to the stock AT&T 4.2.1 ROM? I looked on sammobile and couldn't find it.
You may be able to downgrade to 4.2.1 using mobile Odin but you would need root access on your current ROM.
Do you have knox on your phone? If you do, knox may interfere with obtaining root. Also, tripping the Knox counter will void any warranty left on your phone.