Without knowing model numbers it's hard to say exactly.
Cable Modems are normally connected via coaxial and then have one or several available RJ-45 Ethernet ports for connecting to your network, devices or in your case other routers.
Could you please clarify if this is a existing router that you own that you want to connect to the modem for more ports or extra connectivity, or is it one that you have received from your ISP with the Modem?