A bit more info on this one.
What external router are you talking about?
How is it connected to the TC?
What is the IP internally of the router?
What mode is the TC running in?
Basically if the TC is a router the new secondary router, should not be running as router at all.. it should be setup as bridged device.. that is router bridge not wireless bridge.
This is easy on any router.. bypass the WAN.. set up router IP to match the TC but outside the dhcp scope. Turn off dhcp server in the router.. plug LAN to LAN.. voila, the secondary router is now a switch working as it should.