Networking – Home network with two isolated separate subnets, running on cablemodem/router and WRT-router

The Linksys and the 2Wire can do DHCP on their own LANs at the same time. Let the Linksys router receive an IP from the 2Wire, don't assign it. You don't need DD-WRT for this (although I'd use it anyway).

|--------------|                                              
|   2Wire      |              |---------------|   Wireless Devices
| WAN=(ext ip) |              |    Linksys    |   (x.x.1.???)
| LAN=x.x.2.1  |---[Switch]---| WAN=x.x.2.??? |     |
|--------------|       |      | LAN=x.x.1.1   |-----|
                       |      |---------------|     |
                       |                            |
            Computers (x.x.2.???)                 Computer1
                                                  (x.x.1.???)

In this setup, both the 2Wire and Linksys have DHCP on. The "???" means it receives a dynamic assignment from its DHCP server: Linksys WAN and Computers get it from 2Wire LAN, Wireless Devices and Computer1 gets it from Linksys LAN. You could pretty much just reset the Linksys to its defaults, then configure it with a LAN address of x.x.1.1 and plug it in. LAN DHCP is on by default. WAN will receive a dynamic IP by default (from 2Wire).

As heavyd said, the address for the modem needs to be on a different subnet than the one controlled by your router.

Just to clarify, the subnet mask shows which part of the IP address is the subnet. A subnet mask of 255.255.255.0 means the first three numbers in the IP address are the subnet.

So in your setup, both the router and the modem have the subnet 10.0.0.x. When you enter the address 10.0.0.1, the router sees that address is on the subnet it controls and never sends packets for that address out on its WAN port.

So, as heavyd said, the solution is to change the router's IP address to any private IP address that doesn't begin with 10.0.0.

For example, you could change your router's IP address to 10.3.3.3 and leave its subnet mask at 255.255.255.0. In that case the router's subnet would be 10.3.3.x. Then when you enter the address 10.0.0.1, the router will see that address isn't on the subnet it controls and will send packets for that address onward to its WAN port.