Access To Modem Configuration
From DD-WRT Wiki
 Case of a Bridged modem
In this case the router itself creates the PPPoE connection to the ISP server; but to work, the connection between the modem and the router must have an IP address. Usually the modem gives an address to the client; but this address will never be used except to access the configuration interface of the modem.
My modem is a Speedtouch 510, is IP address is 10.0.0.138 and it gives the 10.0.0.1 address to the device connected to it. The easiest way to know these addresses is to connect the modem directly to your computer and take a look at your network card configuration. The default gateway is the Modem address and the Card address is the one provided by the modem.
Note: Many modems come configured to use the 192.168.1.x subnet. This is the same subnet that DD-WRT uses for the LAN by default. You need to either set the modem to use a different subnet or set the router's LAN to use an IP in a different subnet such as 192.168.2.1 so that they're not using the same subnet.
So it looks like this.
By default if you type the IP address of the modem you have an "Impossible to contact the server" error.
 The way to follow
Replace the 10.0.0.2 with an IP in the same range as the modem. If your modem is 10.0.0.1, you should use 10.0.0.2. If your modem is 192.168.0.1, use 192.168.0.2
- Go to Administration -> Commands
ifconfig `nvram get wan_ifname`:0 10.0.0.2 netmask 255.255.255.0
and click on Save Startup
iptables -t nat -I POSTROUTING -o `nvram get wan_ifname` -j MASQUERADE
and click on Save Firewall
- `nvram get wan_ifname` gets the WAN port of your router automatically. If you wish to enter it manually, you should run echo `nvram get wan_ifname` to get your WAN interface name.
 Alternative way
An alternative way to execute the above commands on startup if you have JFFS enabled is the following:
- Log in via ssh/telnet
- create a file with the following content:
#!/bin/sh ifconfig `nvram get wan_ifname`:0 10.0.0.2 netmask 255.255.255.0
- save it as "/jffs/etc/config/modem.startup"
- create another file with the following content:
#!/bin/sh iptables -t nat -I POSTROUTING -o `nvram get wan_ifname` -j MASQUERADE
- save it as "/jffs/etc/config/modem.wanup"
- make them executable:
chmod +x /jffs/etc/config/modem.*