How do i unlock an unlocked iphone from Tracfone that has never been used on tracfone before?

Okay so in January 2017 i bought the new iphone 7 plus, i was starting out with a new carrier: AT&T but i havent gotten my sim card or set it up (i use prepaid) and at the time i used StraightTalk/Tracfone so when i got my

phone i couldnt wait to set it up but it needed a sim card to work and get in The home screen so i just used my old straight talk sim card to get into the phone so i could use it until i started using AT&T prepaid.

well im guessing tracfone locked my UNLOCKED phone to their network since i put that sim card in because when i went to finally activate my new service with AT&T it put me through activation lock...so soon after that i went to the apple store (it was obviously still under warranty & i didnt know of this problem at the time) and they couldnt figure out what was wrong with it so they just gave me a replacement phone (iphone 7 plus).

i have had the phone for over a year now and it has been working on AT&T Prepaid fine but i got a new one so i gave it to my mom. well i factory reset it for her so she could use it as a new phone and she tried putting her sim card in it (she also has AT&T prepaid) and now it is doing the same thing it did to my first iphone 7 plus and every time she puts a sim card in it (i put my ATT one in too) it goes straight to Activation lock again!! and this is a replacement phone so i dont know how it carried over to being locked to tracfone again

my mom and i went to the apple store again and they said it is locked to trac fone and i have never once used any other sim card or carrier other than AT&T so how is that even possible ? Im just stressing because i have a phone that can basically only be used as an ipod until i pop a Sim card in it than its says it not supported even though i literally used this new replacement phone for months on AT&T prepaid. please help i paid over $800 for this phone brand new!