This article describes how to add port mapping rule on Gateway, Port mapping allow you to redirect traffic destined to one or more ports on your WAN IP to an internal host.
Go to Firewall > Port Mapping and click ADD button to create a mapping rule.

For example, if you went to open the port 80 for only one of your device 192.168.13.1,you can add it like below:

if you have two or more devices(192.168.13.2&192.168.13.3 in this example) want the same port to be opened for a certain service, then you will have to use different External Ports.For the Internal Port, please put in the actual port number(80 in this example), then create different Service port numbers for the two devices(like 9003 and 9004 in this example).

After these configuration, you can access the two devices using different External ports(Service Ports). In this case, you can use WAN IP:9003 to access 192.168.13.2 and WAN IP:9002 to access 192.168.13.3.
If you Port Mapping fails after you have done all the configurations above, please refer to the following steps for further trouble-shooting:
- Check if you can access that server in the local network. If you are unable to access the server in your local network, please check settings of your server.
- Check WAN IP of the Gateway. Verify the Gateway is getting a public IP address.