Nokia Lumia 1520 Unlock problem

Hi,

I don't live in USA, I live in Venezuela, i went 1 week to visit my cousin in Miami, I bought an AT&T Lumia 1520 which the seller on eBay claimed was "AT&T unlocked" The seller is Phonestore USA.

I could see that the cell phone works perfectly, the only thing i could not prove was the phone with my line because the chip was bigger and I decided to try it in my country when I returned. 

When i return to my country, I bought a new chip and when I turn on the phone it shows me on the screen that the chip I am placed can not be used and that I have to put an unlock number of the AT&T to be able to use the phone.

I contacted the seller and they told me that all the phones they sell are unlocked, they told me to try to get the unlock number from the AT&T page because they do not have it. I made the request through the following link https://www.att.com/deviceunlock/request/#/nonattunlock. , It showed me the message - "This device isn’t eligible to be unlocked now because all of the installment payments have not been paid."

 I can not return the phone because the 30 days warranty given by the seller has already passed, and from Venezuela we have many problems right now for international shipments.

How can I solve this problem ?, the seller tells me that he does not have the unlock number of the device and that device was unlocked, I do not think the seller has stolen equipment if he has more than 155 thousand products sold on Ebay with 99.1% Positive feedback.

I would be grateful if you could help me.