Who is your ISP? To use your own router without getting double NAT, the Technicolor gateway(modem/router) needs to be put in bridge mode. From the manual it seems to be controlled by the configuration file the ISP loads on the device:
"Your gateway can be configured to provide connectivity between your cable company and your home LAN in any one of three Networking Modes: CM, RG, and CH. This mode setting is under the control of your cable company, who can select the mode to match the level of home networking support for which you have subscribed" ... "With CM Mode, your cable company must provide one IP address...each PC you connect from their pool of available addresses.".
IMO it would probably be easier to swap to a phone modem, AKA eMTA.