In STRIX game server, only clients in a same room can communicate. No feature is available to send a message to the entire master server.
Strix Unreal SDK allows you to join multiple rooms simultaneously using the channel feature, so you could create a room that all online players always join and send an RPC to all its room members. If you do so, however, please be aware that the capacity of the room caps the concurrent connections.
You could run a server separate from your STRIX game server to send messages. For example, Strix Messenger server allows you to send messages regardless of joined rooms as well as to broadcast to a large number of clients.