FULLY PAID FOR REFERBISHED IPHONE CANNOT BE UNLOCKED

I upgraded my line to the 8 plus model about a week or so ago. My old iphone 6 plus was sent to me refurbished from AT&T about 2 years ago after an insurance claim. It has been completely paid off and under no payment installments which is why I was able to upgrade my line. I went online under my account and used the unlock link and received the follwing error message " Looks like the imei number you entered dosn't match an at&t device."  I've had At&T for at least 9 years and never any other carrier. ALL of my phones come from them. In the settings of the 6 plus under About, the Carrier even says AT&T 31.1. When I put an AT&T sim in, it works great. When I put another carrier sim in, it doesn't allow the sim to be used. I called customer care and after about 45 minutes the rep could not find the phone under my account by the imei (35 204707 230317 0) or the phone number is was tied to ending in 8227. He told me to take it into the at&t store and have them unlock it & reminded me that they work on commission and if they tell me I have to pay to let them know that I was told by the call center to go in and have this done..c'mon now seriously??? When I did this, they told me they could not. I cannot believe how much run around I have gotten with this company after years on top of years as a loyal customer. I am hoping someone here can help me. I've noticed a few people saying to contact  @ATTMobilityCare but I'm not really sure how to do that. It has been a whole week with no resolve to this. I'm really at the end of my ropes with the customer service I have received & ready to take my business elsewhere. This issue is only one among others. 

I have 2 phone lines & just opened an apple watch line upon upgrading. They were running a deal on getting my 2nd iphone 8 plus free for buying 1. I was denied this offer when upgrading even though I was ***willing to open a new line to use it***. I upgraded my first line, then waited a few weeks to upgrade my 2nd line & add the watch on another line due to this. The rep then told me I could have done everything together and got the 2nd 8 plus free (that there was a way to do it but her co worker wasn't familiar with it), so apparently they screwed me again now I'm stuck with 2 iphone 8 plus  installments & couldn't take advantage of the buy one get one offer due to the worker's lack of knowledge of how plans and offers work? This is all ridiculous to me. I have years doing business with this company & definitely do not feel valued as a customer for all of my time and money spent on their products and horrible service.