The Guacamole server needs to be reachable from the internet, but the desktops you're connecting to only need to be reachable from the Guacamole server. That he

The Guacamole server needs to be reachable from the internet, but the desktops you're connecting to only need to be reachable from the Guacamole server. That helps with some but not all of the situations where desktops aren't reachable with a public IP.