You have to join through the proxy!
What to do with a OnlyProxyJoin plugin is already on it
OnlyProxyJoin should be installed on each sub-server and the proxy server.
Is it…
Hm. Unfortunately I can't help there. If you have a VServer you can simply close the subserver ports with UFW btw
Okay I'll give it a try…
So just install UFW "apt-get install ufw", then "ufw allow 22", "ufw allow ssh", "ufw allow 80", "ufw allow 9987" and "ufw allow 25565", then "ufw enable" and it should work. Make sure your SSH port is unlocked.
It must of course not be installed on the proxy, only on the sub-server. You log in via the Poxy.