Unable to reassign personal modem MAC address to my new xfinity account

Xfinity is not able to unlock and assign my personal modem MAC address to my newly open account. The modem was used with a previous account, but this account has been closed. I was told by xfinity agent they would be able to unlock or unregister the modem MAC from the closed account and re-assign it to the new account but as suspected they have not been able to do it. I think the agents I have been talking to simply do not have access to the proper back-end system where this process (re-assigning modem MAC) is done. Can someone from level 4 tech support please email me reply to this posting so we can get my internet service working.