In order to open ports, access router's WebUI.
- Navigate to Network -> Firewall -> Traffic rules.
- Scroll to the bottom of the page, where you will find Add New instance section.
- Select Open ports on router type, enter name, select transport layer protocol(s), enter your desired port number or a range of ports.
- Press Add.
- An additional configuration window will show up, where you can further customize your rule.
- Once done, press Save & Apply.
To check if the port is open, go to this site, enter your public IP address, port number and click Go.