hello,
After logging in to the router,in the navigation tree on the left, choose Forward Rules > Port Mapping Configuration. In the pane on the right, click New. In the dialog box that is displayed, set the parameters related to port mapping, as shown in Figure_1.

Modify parameters.
Click Apply.
Configuration Example
Enable the packets sent from the WAN side to the ONT whose the destination WAN port number is 2000 to be forwarded to the LAN-side PC whose IP address is 192.168.100.20, and the port number is changed to 3000.

I hope it helps. Thank you!