I can't access the Internet if the cable modem is connected to a router. What am I doing wrong?

If the cable modem works fine when connected to the network interface card (NIC) in your computer, then most likely the problem you are having is with the configuration/installation of the router. Look in your router manual or contact the manufacturer for assistance.

The cable service provider needs the cable modem MAC (CMAC) address and the Ethernet MAC (EMAC) address from the computer's NIC. They use the EMAC address to allow your computer to acquire IP address information from them. By placing a router in the mix and connecting the cable modem directly to its WAN port, you are changing the EMAC address being used. You will have to contact your cable service provider and give them the MAC address of the WAN port on the router. They will then add this address to their list so that you will be allowed to access their cable network with any computer connected to the router.