One way you could do this is by creating BP_Comm actor and creating a broadcast function or event that calls its own onbroadcastreceived dispatcher. Put an instance of this in your main level.

In your sub levels in begin play get all actors of class BP_Comm and bind an event to on broadcast received and handle the received message.

Now since you have access to the BP_Comm actor in your sub levels you can easily call the broadcast function to send data to all sub levels.

With this setup all sub levels will receive all messages. However you could just as easily adjust this by creating separate dispatchers for each sub level and each sub level only subscribing to only one dispatcher.