Since you are using cable modem SB8200 you can access internet from only one ethernet port on the modem so if your Xbox have ethernet port you can connect that particular device to the modem and check for the internet or if you can have router connected to the modem you can connect Xbox to the router Ethernet port or to the router WIFI network. Even though you have 2 Ethernet ports on the back of the modem , you will be able to connect one device at the time and the reason why its because the modem is getting just 1 valid IP address
If you want to use both ports at the same time you will need to contact your ISP and request a secondary IP Address