Are you using my build, just clean OpenWRT or using GL-iNet’s?

Either way you can quickly find out what is going on.

Go to Services -> UPNP the check the Enable additional logging Box.
Then in the MiniUpnp ALC section add a new line then put in a lable followed by the IP of the system in question for example 192.168.1.166/24 making sure the ports are set to 1024-65535 then make sure it is set to allow. Then click save and apply.

Once done you can then check the system logs to see where the issue is, if it can not pick up the external IP then go to UPNP area again select advance tab and under STUN add

STUN Host: stun.sipgate.net
STUN Port: 3478

Save and apply again.