"Connection lost because server license can not be validated"
"Server version is too old for command"
"Certificate invalid date"
If you want to use ts3/ts5 client to connect teaspeak server, you will need a Protocol Key License.
Send @world a personal message, he will help with that
In case you dont want to use ts3/ts5 client, then you can use TeaSpeak client, to connect your server without needing a protocol key, like @BIOS said before