This article explains how to connect to the lobby server from other servers. It is important to know if a compatible Mine-Home game is used or not.
With a Mine-Home game
For this case, there is the appropiate help article here.
Other cases
For all other cases an external plugin must be used, which adds a command like /lobby. For this the following plugins are recommended.
These must be configured accordingly to be connected to the appropriate server with a command.