By default, when launched, the TN3270 mainframe display connects automatically to the enterprise server associated with your project.
If you are using the embedded Rumba display and it is not configured to connect automatically, when you launch the display you need to connect it to the server manually by clicking the Connect button .
This is the behavior when you are trying to connect to a server and the display is already in use and is connected to a different server:
Closing Rumba Desktop disconnects all active sessions.