Iphone 6s and Tracfone

I gave my mother my old iphone 6s the other day. Her carrier is Tracfone. I had originally bought this phone from

Apple.com unlocked with the intention of using my Cricket Wireless provider. That being said, Cricket is a division of

AT&T, so I believe that even though I got an unlocked phone, it is a GSM?? which is geared towards AT&T.

So, anywho, I gave my Mom the phone, she bought a BYOP Sim card kit AT&T compatible for the Iphone, set it up

through Tracfone and she cannot get any service on the Iphone at all in the vicinity of her home (now, my Dad's cheapo LG tracfone works fine) but the Iphone....nuthin. So we called Tracfone and they said that the LG Fiesta phone that she was using prior to switching over was a CDMA.....we are confused now.

I guess my real question is this: If the unlocked Iphone 6s that I originally bought is supposed to be GSM (AT&T compatible) can if even be used with a C type of SIM card?? Is this really a SIM card problem or is it a hardware

issue and we would have to get an Iphone that has a CDMA whatever built into it??

Thanks for any suggestions!!