Are you getting IP addresses via DHCP or static? Your router should be set up to issue dynamic IP addresses or you can set it up static. Your ISP will send you a different IP than your router will, usually not anywhere close to the same range as your router. So, if you use DHCP, you should be able to connect directly to the modem with no problems. If you have static addressing, you can't, you either have to go DHCP or change your static IP when you don't use the router. Check your DNS settings and see what they are.