Good job finding it.
When running two modems or routers, it is best not to use the WAN connector on the inside network. Just use the LAN switch ports. Ok, you loose one port for the uplink to the other router but it makes life much easier.
Also, if you use the DHCP option in the router, make sure you only run it in one router. Otherwise you might experience IP address clashes if the hand out address in the same address range.