Arris surfboard SBG10 MAC address not recognized by Xfinity

I’ve been renting a modem from Xfinity and finally purchased my own. I bought a new Arris Surfboard SBG10 modem/router combo from Amazon. It was new in packaging. I tried to get the modem activated through the app and it never worked. I called a customer service line for help and they said their system didn’t recognize the MAC address of the modem and referred me to an in person store. The person at the store was totally useless and said she made a ticket for me but that was almost a week ago and I haven’t heard anything. I really don’t want to pay another month for the Xfinity modem rental. What should my next steps be? Thanks!