generally you need to set your router to forward all requests to your linux server. This is usually called the dmz host. This will allow your linux server to be accessable for all services. The better option is to only forward ports you need (80 for web server, 25 for incomming mail, etc). This is called port forwarding, or pat, or reverse nat. This way only the services you want will be exposed.
You will need to have your linux server on a static ip address for this to work (or get dhcp to hand out the same address each time - called a reservation)
once your router is doing this, your linux server will respond fine.
Hope this helps, if not feel free to ask for further help