Unlock help

I bought a Samsung Express Prime 2 back in late December on Craigslist. Used to be you could use any sim on a GSM phone if the carrier was also GSM (I know there are exceptions. I have a Verizon phone that can be used on GSM and I did not have to go through any process to unlock it). Sadly when I inserted my T Mo sim it was locked. ATT would not unlock because it was registered on another account. I worked with the seller and ATT but no luck. Now that 6 months went by I tried again. Well ATT let me submit a request to unlock, which they wouldn't back in Dec BUT my request was denied because it had not seen 6 months of PAID service. In fact it has not seen any service other than being activated by the previous owner for about two weeks. I would like to be able to use this phone, but not on ATT, or sell it in which case it would be worth more if unlocked. I've seen posts of people being helped by PMing someone @ATTCARES. How and who do I contact to get this done?