Network is going to local and internet to local only.How do i fix this?

I have comcast high speed internet,telephone,and cable(triple play).This is how everything is hooked up

Comcast to wireless modem

wireless to computer and wireless router

wireless router to other computer and xbox

Idk if the modem is bad or what put when i try to play my xbox on live my network changes to local and internet to local only.If anyone can help me please do so.Thank youNetwork is going to local and internet to local only.How do i fix this?
Can't connect modem for computer AND router, modem can have only one user.

Connect router input port to modem output port. Router usually has 4 output ports, so all your internet using machines should be connected to router output ports

1.router - computer

2.router - x-box

3.router - printer etc

The primary purpose of the router is to distribute internet to several machines

Modem is the device for transmitting data between computer and Internet on telephone lines