You don’t state it, but I assume your Windows 2016 server is running Hyper-V to run your VMs.
You also don’t state which port(s) are being forwarded.
Also missing is the info from where are you testing? Internally, from the Windows server GUI?
Or are you actualling trying to use an internal LAN IP from the Internet, eg via a Smartphone Hotspot to test? (Internal LAN IPs do NOT work over the Internet!).
AFAIK, Guacamole needs a https access. In your LAN screenshot, you are using
Wheras in the Port-Forwarded version, you’re only using http, not https…
It depends if you’re forwarding both (80 & 443) or just a single port, but NethServer has no way of knowing that… (Possible issue).
My 2 cents