1. Make sure first no other devices connected to the modem at the same time as some modems has an Ethernet port along with a USB port so, you might think you can use both at the same time & connect the PC through USB with the console through Ethernet & that's incorrect so, we need to disconnect the PC first.
2. Turn the modem off then turn it back on after couple of seconds (by turning the modem off I mean to unplug it from the wall outlet even if you have a power button for it), then go on the Dashboard of your Xbox 360 console to “System” area then “Network Settings” then "Restore to Factory Defaults" and when it ask you to Apply Settings select "Test Xbox Live".
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3. If it's still not working we need to get the Physical Address of your PC to put it on the dashboard of your Xbox 360 console as some cable companies are preventing any unknown MAC Addresses from connecting, to get the Physical Address of you PC connect the PC only to the modem then follow these steps here, the physical address will look like this 00-E0-98-C1-88-C2.
4. Go on the dashboard of your Xbox 360 console to “System” area then “Network Settings” then “Edit Settings” then “Additional Settings” tab then select “Advanced settings” & put the Physical Address of your PC without the dashes on the “Alternate MAC Address” part and select "Done", when it asks you to Apply Settings choose “Test Xbox Live”.