Change default vnc-server port in Catalina

Configure port mapping in your router to take incoming traffic on a different port to go to port 5900 on your mac.

In my environment, after changing the public port from 5900 in the AirPort Utility settings, I was able to share with the changed port.

A better option is to configure the router to point to a custom SSH port on the Mac. First SSH into the Mac, then use your VNC viewer to connect to the VNC port on the Mac since VNC is not secure. To make the SSH connection even more secure you could configure to use keys instead of passwords to connect.

Here is an old article from 2004 which explains very nicely how to configure this type of connection on a Mac. While it mentions using third party apps, I do not believe they are necessary today. I'm referencing it because it explains the SSH connection more clearly than most other articles:

http://preserve.mactech.com/articles/mactech/Vol.20/20.07/VNCOverSSH/index.html

Here is another article for setting up a VNC connection over an SSH tunnel on a Mac:

http://preserve.mactech.com/articles/mactech/Vol.20/20.07/VNCOverSSH/index.html

Here is an old Apple article about how to configure macOS to allow remote access to the server including Screen Sharing:

https://support.apple.com/guide/server/allow-remote-access-apd55d36a80/mac

Ports on your computer are not "open to the world". Your computer is on a private network, the world can not route to your computer. You were correctly advised how to configure your router. Everyone in the world who uses VNC uses port 5900, but no one has access to every user's port 5900 on their computer located behind a router managing a private network.