How to reach cable modem

I tried to follow these instructions:
Connect to Modem
I still cannot reach the login page. I made the changes and rebooted. If I connect my laptop to the modem, it grabs an IP address and I can get the page on 192.168.100.1. These are the relevant bits of my config that I created through the luci interface based on the instructions. My interface is named 'eth0.2' for ipv4 and ipv6 as that is what the 19.07 image set them to when I first booted it.
/etc/config/network

config interface 'modem'
        option ifname 'eth0.2'
        option proto 'static'
        option netmask '255.255.255.0'
        option delegate '0'
        option ipaddr '192.168.100.10'

/etc/config/firewall

config interface 'modem'
        option ifname 'eth0.2'
        option proto 'static'
        option netmask '255.255.255.0'
        option delegate '0'
        option ipaddr '192.168.100.10'

Previously I was using ddwrt and was able to get to the modem interface without making any specific changes to the default setup.

That would be invalid for /etc/config/firewall.

Could you please illustrate what would be valid?

I use 192.168.2.x for my network. The modem is always at 192.168.100.1.

Ideally (though it is not absolutely necessary), the modem will be set for bridge / passthrough mode. This is the default or only mode for modems that have only one Ethernet port, but it needs to be configured on modem/router/gateways that have multiple ports and WiFi.

With default settings of OpenWrt, the WAN interface would submit a DHCP request to the modem, which it bridges unchanged onto the cable. Then a DHCP server at the cable company answers by issuing a public IP to your router.

Again using default settings, the route to the Internet, which is every IP outside your LAN (including by default private IPs), is via the WAN port. This means that as long as your LAN is not 192.168.100.X, should you type 192.168.100.1 into a browser, the router considers that to be a place on the Internet and thus sends it to the WAN port. But the modem recognizes the packet as its internal address and the modem serves you with its GUI page.

All this happens with a default configuration of OpenWrt or any other household router. Though the role of IPv6 may be messing it up, as though the cable company may route IPv6 for you, the modem's internal server is almost always going to be IPv4.

Now if you are still using the modem/gateway as a router, it will DHCP your router's WAN (and anything else connected to its LAN ports or WiFi) a 192.168.100.X IP so then 192.168.100.1 is something that is explicitly routeable through the router. Again though if you have a dual IPv4 IPv6 setup the PC may be trying to use IPv6. That is something I have no experience with.

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