What I needed to do was to create another isolate in the main isolate to communicate using ReceiverPort() and SendPort() between FCM isolate and main isolate.

the new isolate:

ReceivePort? _receivePort;

void startReceivePort() {
  // Unregister the previous SendPort if it exists
  IsolateNameServer.removePortNameMapping('background_send_port');

  _receivePort = ReceivePort();

   IsolateNameServer.registerPortWithName(
  _receivePort!.sendPort, 'background_send_port');

  _receivePort!.listen((message) async {
    // print("Received message from background isolate: $message");

if (message is Map<String, dynamic>) {
  ChatMessage chatMessage = ChatMessage.fromJson(message['message']);
  String action = message['action'];

  if (action == "saveMessage") {
    // Save the message to the database in the main isolate
    await saveMessageToDatabase(chatMessage.chatId, chatMessage);

    bool chatExists = await checkIfChatExists(chatMessage.chatId);

    if (chatExists) {
      ChatController chatController = Get.find<ChatController>();
      chatController.addMessageToChat(chatMessage.chatId, chatMessage);

      int totalUnreadMessagesCount = await getTotalUnreadMessages();
      int totalUnreadChatsCount = await getTotalUnreadChats();

      // Push a notification with unread messages data
      NotificationService().pushNotification(
        id: 100,
        body:
            "$totalUnreadMessagesCount unread messages from $totalUnreadChatsCount chats",
        title: "Conversations",
        category: NotificationCategory.Message,
        payload: {
          "screen": "chat",
        },
      );
    }
  }
}
  });
}

// Unregister the SendPort when the app terminates to prevent memory 
leaks
void stopReceivePort() {
  IsolateNameServer.removePortNameMapping('background_send_port');
  _receivePort?.close(); // Close the ReceivePort
}

You will need to call startReceivePort() before runApp(const MyApp()) in main.dart and stopReceivePort() in the dispose() in your main stateful widget of your app and voila!