Cable modem and router in series

[Casa no Vale]

Did not have a problem with installation at another house, but Xfinity doesn't seem to be playing nice here. I can't get remote access via the phone app. I keep getting a "can't connect" error, and I think its because I essentially have two routers in a row, the first being my Xfinity cable modem (which is also a router), which is connected to a Netgear Nighthawk router, which I use as my primary router.

Previsouly, the Blue Iris server configuration on my phone basically just ignored the cable modem, and it worked fine. But that was a different ISP and cable modem (that modem may have been a dedicated modem without also being a router). Is there something I can disable on the cable modem to reemove the router functionality leaving it as just a cable modem? Or do I need to set up some kind of port forwarding on the cable modem here? Do I forward gthe BI ports, but using the NH router IP address? Or is there some other thing I should be doing with this kind of HW combination?

[TimG]

I have two Asus routers working as a mesh, and was using Virgin UK as my provider. To do that I put the Virgin modem/router in modem only mode. I recently changed to YouFibre as a provider, and had the same issue. For them "modem only" mode for their router was called "bridge" mode.
Can you do that ?

[hspindel]

You need to put your Xfinity modem/router into bridge mode.