Before you begin
The device you are forwarding a port to needs to have a Static LAN IP. This can be assigned on the device, or within the routers UI (Local network tile > Add new static IP lease)
You will also need the ports that the game or application requires to be forwarded.
Log into the routers UI
Select the WAN Services tile
Select Add New IPv4 port mapping
Enter the values as follows
Name: A name for your port forwarding rule, this is simply a reference
Protocol: Depending on the game, if you are unsure select TCP/UDP
WAN Port: The port defined by the game or application
LAN Port: The port defined by the game or application
Destination IP: The static LAN IP of the device
Click the tick to save, and the rule will be set up.
Repeat if the game or application used multiple ports