It took some doing, but turns out that the bridge mode is easier than I figured.
Open asus WL-500gP V2 admin page (admin, admin)
Set Wireless SSID same as main router
Set channel, wireless mode, authentication method, WPA encryption method and WPA pre-shared key the same as on main router
On left side: Advanced Setting/Administration; select Access Point and it will do all the work.
To log into the second router admin page you need to know the ip address given by the main router.
Go into the main router and look for the device list/network map to see what address was given the second router