Whats the routers vendor? e.g. Alink adsl modems need your ip to be set as 10.0.0.1 before it lets you in. If this is the case, you can get this simply by executing as root:
ifconfig eth0 add 10.0.0.1
It will create a virtual interface with the ip address 10.0.0.1 allowing you thru to the router.
Other vendors have similar methods, but without you telling the make / modle etc we cannot help you