To configure your ARRIS BGW210/ARRIS BGW320 with a 3rd party router, please do the following:
1) Open browser and go to 192.168.1.254
2) Go to Firewall > IP Passthrough
3) Change Allocation Mode to "Passthrough", and Passthrough Mode to "DHCPS-fixed"
4) Select your router from the drop down menu for Passthrough Fixed MAC Address