[tsac]

Normally any device on the local network can access a internal server using the correct IP address. You will need the IP the server has on the lan side of the router.
Generally the address will be something like 192.168.1.65. the Router will also have a gateway address used to exit to the internet. the Internet address will be an IP assigned to your service such as 62.33.99.33 The "local" devices only go via the internal IP addresses when going to your server but "hit" the gateway address when going out to the internet.