Unlock AT&T

I just upgraded my iPhone and reset the iPhone XS to factory. I was approved an unlock request from AT&T. Is there anything special that I need to do to unlock it? I want to sell it as unlocked. AT&T's email states to remove you sim card and insert a non-at&t sim card and complete the setup process.

Will that work if the phone has already been reset?

Thank you for your answers.....