Modem Connection/Provision issues

I recently bought a Arries modem SB8200 to replace my old modem and could not provision it through Xfinity.  Thought the modem might be the problem so I bought a Netgear CM1000v2 and same thing, Xfinity can't activate it.  Each time it has happened I been able to connect my old modem back with no problems.  Has anyone had this issue?  I have been told by Xfinity support agents that perhaps I should get an Xfinity modem??  Don't understand why being that I am able to connect back with my old one.  It's been a frustrating experience spending numerous hours on the phone with a support agent, if I am able to reach one to no avail.