Timing out connecting to VNC Server on Ubuntu from Windows

Firstly, should it be possible for me to connect to VNC Server on an Ubuntu device from a Windows device with no paid license?

I am already successfully connecting to VNC Server on an OSx device from the same Windows device successfully.

I have VNC Server set up in Ubuntu but when I try to connect with VNC Viewer, it just takes a while to attempt to connect and fails with a 'Timed out waiting for a response from the computer'.

I have no idea what the problem is and I have wasted many hours over the past few years trying to get this working (it used to work fine). The device I am trying to connect to is on, configured, on the network with no firewall restrictions.