I currently have an AT&T Iphone and have a Sprint s7 edge. I want to use my s7 edge with my AT&T account and i found a setting called network operator and it says that the network mode can be changed to GSM/UMTS. I was wondering if i could use AT&T on it with my sim card which is the same size as needed and how to do it and was wondering if it would cost me money to do.