I would go back to default settings with router. If your ISP modem has an IP address of 192.168.0.1, then use .1.1 and change the DIR-655s IP address IF needed.
If the modem is bridged, then use the PPPoE settings on the router to establish the connection if your using DSL and a PPPoE connection. Make sure ALWAYS ON is selected. You should need to change the routers IP address if your using bridge mode on the modem.
HW set up should be as follows if your going to use an external router:
ISP Modem (bridge mode)> DIR-655><Network Switch><All wired devices to the switch.