Fiber modem and router are up, but no internet connection

ISP setup involves a Comcast modem (F50 model), a D-Link D600 router, and the Ethernet cable provided with the router plugged into LAN1 on the modem and the WAN port on the router.
The local network itself seems fine—my laptop connects via Wi-Fi without any issues—but there is absolutely zero internet access. To test it further, I tried connecting my laptop directly to the modem via Ethernet, and still, nothing. No connection at all.
Does anyone else here run this specific hardware combo? If so, could you walk me through the proper configuration to get everything running smoothly? Also, a side note: the default credentials for the modem (admin/admin) aren't working; I can only access the D-Link settings, which limits how much I can tweak.

So, what's the word from your ISP's customer support?

They’ll check the modem and the router, see that everything looks perfectly fine, and still have absolutely no clue where the actual issue lies.