Troubleshooting a network connection to the Internet can be quite challenging. Your approach should be to eliminate potential sources of the problem one by one until you find the specific problem you are having.
If you are unable to connect to the Internet after you have installed and configured an Internet Sharing hub, try the following steps:
Remove the hub again and return your configuration to the way it was before you installed the new hub.
Return one Mac to the condition it was in when you were able to access the Internet.
Check to see whether you can access the Internet. If you can, that means your modem is working properly and the configuration information is correct.
Attach that single computer and your modem to the DHCP hub again. This removes potential causes of problems that are due to the interaction of devices on the network.
Repeat the hub configuration and verify that the settings match those you used to connect the single Mac to the Internet.
Set the TCP/IP settings on the Mac to use a DHCP server.
Try to connect to the Internet. If you can connect, add your other machines back to the network. If the problem recurs, you know it is related to interaction among the devices.
In some cases, you might need to configure your hub to use media access control (MAC) address cloning. A MAC address uniquely identifies each node on a network. In some cases, you will need to clone, or copy, the MAC address of one of the computers on your network onto the router. See the instructions that came with the hub to learn how to configure MAC cloning on your router.
If you still can't connect, call your ISP for support.