This is more a networking issue. You will have to configure your router with appropriate port forwarding. If your ISP does not have static IP's you may also need to set-up some dynamic DNS service.
The NAT traversal required to connect to external networks requires a static IP outside the 192.168.. or 10...* range. This is typically assigned by the ISP DHCP server to the external facing MAC Address of the router.
The port forward settings are shown here: