I have found that the plugin works if the game server and the website on on the same server, but if the game server is somewhere other than the website then plugin says you are offline.
I suspect this has something to do with apache or nginx setup, firewalls or whatever. I am not sure how to word this actually ask for help in the matter as I am having the same issue.
I have a game server on the same machine as my website and the plugin works beautifully, I have another game server on another machine and the plugin says that game server is offline.
Maybe my articulation can elucidate a response from someone who knows what the issue is and help us to resolve it. I doubt it’s the plugin.
My site is at https://www.kreezcraft.com if you would like to see the plugin in action for the two locations.