Port Forwarding for VNC port 5900

I have been trying to get rid of teamviewer, switching to VNC protocol on my linux compter (Kubuntu 18.04 LTS), and I can successfully connect to my internal ip address from any vnc client within my home network but I want others to be able to connect to my computer from anywhere, like teamviewer does. I tried forwarding port 5900 from wan to lan and made sure to choose my correct internal ip of the device i want connections to.

The port I have added so far is:
Name: VNC

From any host in wan
Via any router IP at port 5900

Forward To
IP, port 5900 in lan

but at sites like canyouseeme.org doesn't show that it is setup properly, i added it and then rebooted router and still not shown that its working. Am I missing something here?
I've managed to get port forwarding working before I started using an open source router, but even then sites like canyouseemore.org wouldn't recognize it even when the device it was setup for saw it so im not sure how reliable they are.

Thank you

Safer way would be to run an OpenVPN server on the router with certificate authentication and connect thru it.

Never heard of doing that, I do however have OpenVPN set up already as my entire network is connected thru it. I had just thought, maybe the VPN i have setup is whats causing the issue. How exactly would I do as you suggest? and would this interfere at all with the VPN I have setup?