iPhone Device Unlock Error on Device Unlock Portal

Hello,

I am not an AT&T subscriber. I am trying to unlock a former AT&T iPhone that was sold to me as being unlocked. For the past few weeks, using several combinations of PC's, browsers and network connections, I have received the following error on the Unlock Portal after putting in my IMEI, name, email address:

"We're sorry, but we're having technical difficulties with our network at the moment. Please try again later."

I have tried different email addresses, I've tried clearing browser caches, to no avail. I have called AT&T Wireless customer service several times, each time referred to an authorized AT&T store. I went to the store, they said they don't have the resources and pointed me back to customer service. I have wiped and reinstalled iOS on the phone. I put in a two different T-Mobile network pre-paid SIM and keep getting SIM not recognized. These SIMs do work on another unlocked iPhone I have. 

Here are the specs of my phone: iPhone 7 Plus  A1784

Please assist with getting this phone unlocked, thanks in advance.