Your modem RNCM40 is a DOCSIS 3.0 Cable modem with 4X4 channel bonding. We recommend that Preferred 150 customers use D3 8X4 or higher modem and Ultimate customers use D3 16X4 or higher modem. The link provided below will assist with the Cox Certified Cable Modems.
FYI:. I noticed a significant improvement after upgrading my 10 year old cable modem. Even though my plan at the time couldn't fully utilize the additional speed I saw much more consistent speeds and lower ping times during speed tests.