The free version doesn’t place any limitations on whether you are using it on a LAN over the network. It is supposed to do exactly that: provide access to users over any network 🙂
How you have set the software up is exactly one of the ways that we want it to be used. However, as a commercial set-up you should ideally use NoMachine Enterprise Desktop. The EULA places limitations based on the type of deployment.
That said, you will have to manually configure your router, each VM should have its own port, instead of relying on UPnP which is really geared towards the “consumer”. UPnP is intended primarily for residential networks without enterprise class devices, and for those users who aren’t network-toplogy savvy. UPnP provides the “plug in and play” element in the free version so that inexperiencedusers can just install and use it to access their remote PC or Mac.
You can read more about UPnP here:
https://en.wikipedia.org/wiki/Universal_Plug_and_Play
and here:
http://community.spiceworks.com/topic/128257-what-is-the-best-enterprise-router-for-upnp
All in all UPnP is OK for home networks and not a protocol on which you should base your commercial solution that you have created.
A solution, to avoid having to manually configure your router for 25+ servers, could be to evaluate a multi-node set-up. This would make it much easier to manage your users and VM’s, and you would be able to add new nodes for those VMs on the fly. As a bonus, rather than have multiple IPs and ports to manage, you would have a single IP/port for entry because on your Ubuntu server you would install Enterprise
Server. By configuring and setting the necessary parameters on the NoMachine Server, you could then bind users to their respective VMs. Each of those VMs would function as a node with Enterprise Desktop installed.
We can certainly help you to resolve any technical issues, but to do that we would need further information about your network topology which ideally is something that the sales team can help you with along with giving you an estimate of the licenses that you would need for your environment.