Modem Upgrade Install

I downloaded the list of Cox approved cable modems and went and purchased one off the list. Netgear CM500. But it won't recognize it on the activation page. It has a drop down box and says to select the modem I want to install. The Netgear CM500 is not on it and it says if the modem is not on the drop down then it is not compatible on the Cox network. So which is right ,the list I got from Cox.com or the activation page? And how do I activate a modem that is not in the drop down choices? The great majority of the modems on the cox approved list are not on the drop down menu.