Factory Reset

I am having difficulty performing a factory reset on my Galaxy S7 phone. When pressing the factory reset button under settings I get a screen that asks me to log in to my Samsung account. This phone no longer has either a SIM cord or a storage cord and it is no longer connected to the network so there is no way to log in to my Samsung account. Any suggestions about how to perform the factory reset as I want to wipe all the information off the phone before returning it to AT&T?