Okay, this story has a happy ending
Summary: I called Apple last night. They had no idea what the activation lock was but they spent over an hour trying to help me. We wiped the phone from iTunes, swapped SIM cards among many other things. Nothing worked. She talked to her supervisor who also had no idea and suggested I call AT&T.
I called AT&T, they said I need to call Apple and hung up.
I called AT&T back hoping to get someone else at AT&T. The second AT&T rep also didn't know what the activation lock was. They tried manually adding the device to our AT&T account, force activating the SIM card, swapping SIMs etc. Nothing worked. He talked to his manager and suggested I try the process in iTunes. I told him we already did this during the Apple call. He gave me a activation hotline number and said to call in the morning.
It was getting late so I sent a message to the seller and requested that they call the AT&T number that was given. I woke up with a message this morning saying they tried calling but it didn't work. They asked for IMEI, S/N, and SIM number along with the phone number that I wanted to use. They said they would call AT&T back once they had this info. I replied back with the info.
While I waited this morning for an update, I called AT&T for a 3rd time, this time calling the activation hotline. The woman I spoke to was the most clueless of them all. She also had no idea what an activation lock was and didn't even know if I should be swapping SIM cards. After 5 minutes on hold she told me that I may have damaged the 6s because of putting the 5s SIM card in since "they use different technologies." I knew I was wasting my time so I thanked her and hung up.
About 30 minutes later, I got a random call from AT&T and the rep said she had the seller on hold. She wanted me to verify the number, IMEI, and account passcode on my end. She completed a fake activation but that didn't work. After I described to her what I was seeing on the screen, she said that this was an Apple issue and that she was going to hang up and call Apple with the seller still on the line.
20 minutes later, the AT&T rep called back and said Apple was useless(ha!). She said that nobody knew what to do at this point so the ebay buyer agreed to provide the last four of their social and zip code. Weird thing was that I never spoke to the seller and it was the AT&T rep that provided me with the necessary info. Kinda weird to see the rep play middle man.
I entered in the info, completed the iPhone setup, turned off the phone, inserted the correct SIM, and bam! The phone worked with our number. I thanked AT&T and sent a message and high rating to the seller. Super nice person!!! I also told the AT&T rep that 3 other reps and 1 Apple rep could help me but that she did.
We did an iTunes wipe of the device to ensure that it was truly fresh. I then submitted a unlock request as well since the seller paid full price for the AT&T version. Wife is now enjoying her 6S Plus Rose Gold 128gb and I win husband of the year award according to her.
The End