Why is the local server not visible from the Internet?

Question

Domain Name System

The bottom line is this:
There is an RPi on the local network with a server on port 30003. The server is accessible from the local network and I want it to be accessible from the Internet.
To do this, I set up port forwarding on the router and use the Dynamic DNS service because I have a gray ip.
When I go to the address from the DNS service without specifying the port, I see the admin panel of my router. And if you add the right port, then the browser cannot establish a connection.
What could be the reason?

Answer 1

Several possible reasons:
1. Port forwarding is incorrectly configured
2. firewall blocks connections from the outside to port
3. The server itself, listening on port 30003, accepts connections exclusively from local area
The third option can be checked using the command:
sudo netstat -tulpn