There is a switch on the back of Buffalo routers that has the options for On, Off and Auto.
When using the router in router mode as the main router on the network you will want to keep this switch to on.
Off should be used if the router is going to be a repeater or a switch.
Auto should only be used with other Buffalo routers as a switch or router. The unit will auto sense what mode it needs to be in. If you use the Auto mode with the router and Buffalo is the main router sometimes it will not get a new IP address from the modem when using some cable providers such as Comcast. To fix that issue you would have to flip the switch to On and power cycle the unit and the router would be able to get online again.