So your 2 computers are not networked. Do they run thru the same modem/router. Are you sure Zoom recognises UPnP technology. You could try forwarding a port for the comp you're having problems with: http://www.portforward.com/routers.htm If you do the steps are
1. Set up a static ip
2. Forward a port (port 6346): http://www.portforward.com/routers.htm
3. Manual port forward within LW: Manual port forward instructions & sample image & 2 posts after that.

I can't see any other reason at present that you should be firewalled. And yes your Zoom does use an internal NAT firewall. Running 2 should be ok if it recognises UPnP ... I know there was an issue with running UPnP on a lan & I still don't know if this has been fixed yet. So you can run one as UPnP & the other as manual port forward I guess. Try it & see if it makes any difference.