Need help on Samsung Account Lock on S5

I bought a used S5 on Craigslist with firmware G900AUCU4BOF2. I didn't know about Samsung Account Lock so didn't go that far when looking at it. The guy had the box so I wasn't thinking it was stolen or anything like that but yet I cannot get a hold of the seller as of yet. I saw the videos about getting to the accessibility screen but I can't back up and do that on this firmware so I'm assuming that hole was patched. Is there any other way to get the phone unlocked or is it just junk at this point? Would Samsung or ATT do anything? The IMEI is fine. I feel so dumb and appreciate any help.