It’s hard for me to give you exact advice because I don’t know how your local network between these devices is structured, and networking was always my weak spot. But if I were you, I would take a step back from Unity and ROS and first try to verify that the devices/virtual machines can connect to each other via SSH first.
So for your ‘Unity on Windows ↔ ROS on Ubuntu Virtual Machine’ setup, I would first try to SSH from windows to that virtual machine (make sure the SSH port is open) to check if that IP is reachable. This should be an easier problem to tackle.
If you can’t SSH to that Ubuntu Virtual Machine, you might be trying to connect to the wrong IP address, or it is not accessible from outside VirtualBox (check how VirtualBox networking works because I have no clue). If you are able to SSH to that machine, then you know that the Unity-ROS connection should be possible and something’s wrong in either Unity or ROS.