If you are trying to access the modem interface using the router connected to the modem will not allow modem IP to access so in this case we would suggest you to access modem GUI on direct connection without router. Disconnect the router and restart modem for a minute then connect device like laptop or PC to the modem ethernet port then access interface after the modem reboots completely

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

@JadenHutson  even direct connection might not work. seems to be something w the firmware.
 

was not an issue on my old sb6183 - with or without a router

try this work around -

its annoying.

Having the same issue.  I could log into my old sb6183 as well.  Just change my IP to 192.168.100.200 and log into the modem.

I can’t do that with the SB8200.  I've tried shutting the router off, connecting a cable and doing the workaround as mentioned above. Doesn’t work.  I can telnet both ports 80 and 443 so I know they are there.

@scottdi was able to access the gui plugging directly into the modem and

do this ->

Thanks, wish I could, I tried that and it just times out.  I can telnet to port 80 and 443 so I know it’s out there.