Here's the setup: fiber optic cable coming from the street up to your house connects to the ONT(Optical Network Terminator). The ONT breaks out the signals to data, voice and at some point there will be video. On the data side... the ONT gives you a WAN connection that plugs into the WAN connection of the router (DI-604 or DI624 wireless router). From one of the switch ports on the router, that will connect to your NIC.
Hope this helps.