Linksys Router or Gateway Mode

Good review of this feature can work (or not work):

http://www.dslreports.com/forum/r21464704-Linksys-Gateway-Mode-vs-Router-Mode

Hint: use gateway mode if the box actually conencts to the internet. If for a secondary router, set to 'router' mode and do not use the WAN connector.

Also, this setting if your box has it, should be under Advanced Routing.