Can't register brand new modem. Support says it has been "registered to a different account."

Recently upgraded my service to the 2000 Mbps plan and purchased a Netgear Nighthawk CM3000 to take advantage of the faster upload and download speeds as it is on the list of approved devices to do so. When I attempted to activate through the app, it would not accept the MAC address. Contacted an agent through the chat and was told the MAC address was already registered to a different account, though they could not figure out what account. They stated I had been sold a refurbished unit, which is not the case. I informed them that I was certain it was not refurbished, I had just purchased the device brand new and it came factory wrapped before I tried to activate it. I was subsequently passed to two additional agents who said the same thing and advised I buy another modem. After 3 hours, the issue remains unresolved and the Xfinity modem that was shipped to me was temporarily activated. I would rather not purchase another modem only to have the exact same issue. Can anyone tell me how to resolve this and have my modem activated?