There is no port number meaning the port is 80

So suppose your public IP is 1.2.3.4, and GL-Inet router’s WAN IP is 192.168.1.10

You should set up port forward

1.2.3.4:9999 to 192.168.1.10:9999
192.168.1.10:9999 to 192.168.8.100:80