Well the problem with the DPQ3212 isn't that it's not compatible with Cox, it's that it was Cox's choice for their EMTA since they upgraded to DOCSIS 3.0 over a decade ago. Part of their phone service included that modem which could also be used as a gateway for internet services gratis. However, the modems often times weren't returned after phone service was disconnected and customer mistakenly thought it was "their" modem. So if modem is never returned to Cox, they get flagged, or if they get returned in the wrong manner, they get flagged, if it falls into a MAC address range that was marked as decommissioned, they get flagged. Once it's flagged it takes an act from an inventory specialist to re-register it to an account and it's on high on Cox's priority as so many of these are modems that are as far as Cox is concerned, stolen property. 

 If you have a DPQ3212, I would get the serial number and mac and call in or head to one of the retail stores and make sure it registers on your account.

-Thomas Scott