If you're hosting the server through TNServerInstance:
TNServerInstance.serverName
If you're just a client:
The name is sent when you retrieve the list of channels. So just save it to a variable from there. Otherwise, you could add a custom packet to get the name from the server at any point.
But, for debugging, the IPEndPoint of the server should probably suffice, right? You can get that from TNManager.tcpEndPoint.