C7100v isn’t recognized and can’t be activated by xfinity

I recently moved into a new home and when I arrived the landlord had internet setup with xfinity and xfinity’s equipment. I am suppose to start my own service so I purchased a modem/router combo. It was refurbished and had stickers with new Mac numbers over the old ones. Spent hours with tech support, did all the basic resets/ power cycles, tried both old and new Mac numbers but xfinity was never able to see the modem on their end and get it activated. So I sent the modem back and got a replacement. But again, same issues. Can’t get it activated with any level of tech support had an agent come out he couldn’t tell me why it wasn’t working. I’m not great at the jargon of all this, but very frustrated