How to signal to the database that I have lost the connection - Java

How can i make that if player1 loses connection, firebase will get signal, i just need to know how to make firebase get notification that player1 lost connection for example

I use this to check if there is a connection and with this you can do all this, but from here I will not be able to transfer the sign to Firebase that I have lost the connection, because in order to transfer it you need to have a connection:

DatabaseReference connectedRef = FirebaseDatabase.getInstance().getReference(".info/connected");
connectedRef.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot snapshot) {
        boolean connected = snapshot.getValue(Boolean.class);
        if (connected) {
                System.out.println("connected");
        } else {
                Toast.makeText(DuelGame.this, "У вас отсутствует подключение к интернету!", Toast.LENGTH_SHORT).show();
                System.out.println("not connected");
        }
    }

    @Override
    public void onCancelled(DatabaseError error) {
        System.err.println("Listener was cancelled");
    }
});

Once again I will explain more simply. I need Firebase to check if people are online or not, if not then some actions are enabled.