When connected LAN to LAN, everything is on the same network/level (they're both equal; one's not "down-level" from the other). This is why you need to disable the DHCP server on the new router. Everything connected to the new router, both wired and wireless, will simply be passed along to the existing RG.
The WAN port on the new router should be left not connected to anything.
With the DHCP server disabled and nothing connected to the WAN port, the new "router" is now nothing more then a dumb switch and wireless access point.