Honestly, it took me 3 tries trying to talk to Live Chat on ATT until I actually spoke to a Lead (it said Lead by her name) and she was able to provide me with the unlock code. For some reason in AT&T's system it says Lumia 2020 and of course it does not provide an unlock code since it is the wrong device. I even had one person keep telling me that my device was not a 2520, but a 2020. It was unreal. Trying to do it on the online link from AT&T doesn't work, and even a request submitted to the team, didn't work, saying my device "was not supported by AT&T any longer".
The connection seems more reliable than my AT&T one since the LTE seems to stall out at times. However, I believe the tablet is accessing the network on T-Mobiles refarmed 1900MHz frequency, so coverage can be determined from their website.
I finally got my device unlocked, and with T-Mobile I seem to be getting HSPA speeds (it does not say LTE). I am kind of curious if I can force it to LTE (I know the MyAT&T app you can select your speeds) but since it will be on T-Mobile, the app does not work and I can't select my speed. SpeedTest showed though that the T-Mobile HSPA was faster than my "LTE" connection (at least in my area).