What you want to do is plug all the computers onto the router then unplug the router and modem. Plug the modem back and wait about 10 second then plug the router back. Go to one of the computer and type cmd in run to open up command prompt. Type ipconfig /all to see if it received an IP address, subnet mask, default gateway and DNS from the router.
Now ping your default gateway, for example if my IP information was IP= 192.168.1.101, subnet mask=255.255.255.0, default gateway=192.168.1.1, DNS 68.etc.etc.etc then I would type ping 192.168.1.1 to ping my default gateway which is my router. If you get a reply then ping your DNS server, if you get a reply then ping www.google.com
If you can ping your default gateway but can not go any further then that, then you will probably need to do the MAC cloning. Sometime your ISP will record the previous MAC address of the computer and will only allow that computer to connect, so adding a router and bypassing this filter would be difficult if you donÂ’t have the MAC cloning feature in your router. What you want to do is connect a computer that you connected before to the modem and try to repeat what you did to get online, now open up command prompt and do a ipconfig /all and take note of the physical address (MAC address) of that computer, plug every computer back into the router, go into the router configuration and find the MAC cloning setting, enter that computer MAC address into the setting and save. Unplug and plug the modem and router like above.