I purcahsed an Arris Wi-Fi Cable Modem ( model No. SBG8300), as it said it was compatible with Xfinity. I've attempted to set it up on my xFi app, but when I get to enter the CM MAC address, all I have on the modem is an HFC MAC address. When I enter it in, or even use the camera to scan the bar code, it rejects the address and asks for me to enter the number.
You can't use that with the Xfi app. It has to be one of their rented gateway devices, not retail / customer owned.
You stated: " I purchased an Arris Wi-Fi Cable Modem ( model No. SBG8300), as it said it was compatible with Xfinity. "
"Compatible with Xfinity" simply means that it will work on their system. It does not specifically mean that it is compatible with Xfi / the app.
The only RENTED devices that will work with Xfi are the ones that state "Xfi eligible" here on the chart on this page (scroll down);