How can I access login page for SB8200 modem if it won't load?

This seems to be a common problem that probably depends on the behavior of your router. The modem’s configuration page should be accessible at: 192.168.100.1

It works for me behind my ASUS RT-AX58U router. My LAN network ID is not 192.168.100.x/255.255.255.0 but I’m not sure if that has something to do with it. Unfortunately the ARRIS device does not permit us to change its listen IP address to something other than 192.168.100.1 (a peculiar and most irritating limitation).

Since you were able to access the interface page without the coaxial cable connected, but not when the cable is connected, we recommend trying the workaround method on your computer.

Please refer to the link below for instructions on how to access the web interface using this workaround:

https://arris.my.salesforce-sites.com/consumers/articles/knowledge/Workaround-Accessing-the-Cable-Modem-Status-Page

I would think, for this to work, that there need to be two things in play...

1. Make sure your home network doesn’t use the 192.168.100.x/255.255.255.0 network ID (aka 192.168.100.0/24)
2. Your router must support routing from your home network ID to 192.168.100.1 through its default route (aka default gateway); this is normally the router’s IP address on the LAN side. (Different router brands default to different LAN IP addresses.)

This is what happens on my home network, which does not use the ID 192.168.100.0/24. When I request to connect to the SB8200’s web server (192.168.100.1), my router correctly sees this address as “not on my local network” and routes it through the WAN port, which is where the SB8200 is connected, and the connection succeeds. This is evident in the SB8200 log data, which shows the GUI login as successful from the public IP address assigned to my router’s WAN port.