ZenFone 2E unlock request denied

I have an ASUS ZenFone 2E that I purchased about 3 yrs ago to replace my broken phone. 

I now want to have it unlocked as I now have a new replacement phone.

 I requested for unlock and I got the following:

We can't unlock this AT&T PREPAID device right now Looks like your account hasn't been active for at least 6 months.

Why??

I do have another active phone that I use with my active account with AT&T.

How am I now supposed to use the ASUS Zen and my current phone simultaneously just to get my unlock request completed? 

Very frustrating.