Borrowed knowledge: DHCP passthru on Netgear WGR614 (This may be applicable to other routers). Set up your other DHCP servers range of addresses say 192.168.1.2-xxx.30, now set the routers IP as an address outside that range. Say, 192.168.1.50 Turn off the routers DHCP. Plug your other router into a LAN port NOT the uplink port.
borrowed from here