Did you start here?
www.att.com
I put in an unlock request for an iPhone 11 that was my wife’s old phone and received the email from AT&T within a few minutes.
Ran through he unlock and worked as advertised.
Might be a lot of requests being sent in, but an unlock request (assuming you meet the unlock criteria) should be within a short time frame as in minutes and not hours and definitely not days between when you submit your request and when you get the email notifying that the unlock request was approved.