Hello bobo

                   SB8200 is just a cable modem so it doesn’t have any advanced settings or configuration the GUI or interface and so you can mostly access modem interface 192.168.100.1 can be accessed on the device connected to the modem directly without router, because some router will not allow modem IP to access. so please try connecting any device to the modem directly and before swapping device restart modem for 5mins( disconnect modem from power then reconnect to power after 5mins) then use any web browser to access interface and use the default login credentials to login. For reference please look for the steps mentioned on the link below 

    http://arris.force.com/consumers/articles/General_FAQs/SB8200-Web-Manager-Access