trying to unlock iphone6 but AT&T say's that IMEI # doesn't match a compatible device
I originally bought my daughter an iphone6 back in 2014 when we were still with AT&T and we switch to a different carrier back in 2016. I was able to unlock her iphone6 and my daughter have been using the phone for almost 2yrs under a different carrier. I recently brought in the phone to Apple to get a screen replaced but because the calibration test failed they had to give me a new phone. The new phone that they game me was an AT&T phone because they said that the device I brought in still says that it was an AT&T device. I was told that I just needed to request to get the new iphone unlock through AT&T. I tried requesting to unlock the phone but I keep getting an AT&T error stating that the IMEI entered doesn't match an AT&T device. I had called AT&T customer service and was told that I needed to activate the phone with an AT&T account which I did buy purchasing a prepaid sim card. The phone is activated, however, when I try to request to unlock the device again, it still gave me an error that IMEI# entered doesn't match an AT&T device.
Please help, my daughter has been without her phone for a week now and it's been really inconvenient for me.