Network access to VirtualBox VM from Network PCs

If you just used the default Network settings in your VM, it will be NAT with DHCP. That means you get a LAN IP from your VirtualBox DHCP Server and you can't really access the machine from the outside.

Check out the VirtualBox Manual on Networking: https://www.virtualbox.org/manual/ch06.html#networkingmodes

I guess bridged neworking should be best. You should configure a static IP address.

EDIT: Concerning the edit removing "host-only networking": It's true that you can't see the VM from outside. But you get virtual interfaces (vboxnetX) on your host machine. So if you route it through, it should work. The setup is more complicated, but the VM is less exposed.

You should either expose IP address of VM to external network and NOT make it DHCP, either set up packet routing on PC2 so it will work as proxy (which i've no idea how will you do it on Windows, on Linux it is way easier).

As an option you can setup web-server on PC2 and make it proxying to VM, but again in this case u better off with static IP for VM and not DHCP.

Third option would be combine all of machines into VPN where PC2 will be a router/server. But that is too complex on Windows to set up.