Keep in mind that Port 5900 is an example port.
(Tcp 5900 is used by VNC, if that is what you are testing or trying to make it work, first make sure you have VNC configured and running on your local machine, and you can access your local machine via VNC Client on another Local Machine).
Then you can test from a different connection ( outside) .. Keep in mind that you will not be able to test port forwarding from inside the network.
Needless to say, 69.69.69.69 is an example .. you have to change it to your static public IP.