How to connect Belkin wifi router to Surfboard modem

I am trying to connect my own Belkin AC1200 wi-fi router to Xfinity's Motorola SurfBoard SB6121 modem?

Internet works with no credentials or other setup when my laptop is plugged directly into the modem.

But when I plug the router into the modem, the light on the router flashes blue and then turns orange, and the connection does not work. When I connect my phone to the Belkin wifi, I simply have no connection. When I connect my laptop to the Belkin wifi, it tries and fails to redirect me to a network login page. If I go to the Belkin setup page at 192.168.100.1, the automatic network configuration tool fails and asks me to power-cycle the modem. Doing this does not fix the problem. I have tried to configure the router manually both for dynamic and static connection, but I'm stabbing in the dark at the settings and cannot get it to work.

I also note that the modem setup page shows that DHCP is active on the modem, but there are no user-configurable settings on that page. I also have an Xfinity username and password, but I have no idea where or whether I am supposed to enter these credentials.

This setup (at my dad's house) was working, so I'm pretty sure it can work, but something got messed up and I'm trying to fix. Wit's end here. Any suggestions welcome.