A:Answer I'm confused by your "wireless modem", in my answer I'm assuming you mean it's cellular based.
As a "router" it does need to be plugged into the modem. However, per the directions it can also be used as an "access point", which would not need to be connected to the modem, but would still need a wireless router to send the signal it will receive.
To do what you described you would need two of them. One as a wireless router at the modem, and one as a access point at the Xbox.
