You do not need to do anything special in terms of accessing your modem's status/config page. In my case, I use an Arris cable modem, which has an address 192.168.100.1. My LAN is 10.0.1.0/24, and I have a public IP address on my WAN via DHCP.
All I need to do is enter the modem's address in my browser and I'm there. No special routes, networks, or any other configurations required.