Why my Android Studio app can't reach my nodejs server?

Question

My project involve Android Studio for the implementation of an app running in my phone and a nodejs server. As how you can see in my app code I print the address but never enter in the function onSuccess neither onfailure. Just it get stucked in new AsyncHttpClient().get(url+beaconHash, new JsonHttpResponseHandler(). What should I do to connect my app with the server? Server code app.js:

const express = require('express');
const path = require('path');
const bodyParser = require('body-parser');
const morgan = require('morgan');
const cors = require("cors")

// Import base routes
const indexRouter = require('./routes/index');


// Database configuration
const host = 'localhost';
const dbName = 'foldersdb';

const mongoose = require('mongoose');
mongoose.set('useCreateIndex', true);
mongoose.connect('mongodb://' + host + '/' + dbName);

const db = mongoose.connection;
db.on('error', function () {
  console.error('Connection error!')
});
db.once('open', function () {
  console.log('DB connection Ready');
});

// Init express app
const app = express();

// Enable CORS
app.use(cors())

// Setup logger and body parser
app.use(morgan('dev'));
app.use(bodyParser.json());

// Setup static public folder
app.use(express.static(path.join(__dirname, 'public')));

// Setup base routes
app.use('/', indexRouter);

// Catch 404 errors
app.use(function (req, res, next) {
  const err = new Error('Not Found');
  err.status = 404;
  next(err);
});

// Error handler
app.use(function (err, req, res, next) {
  res.status(err.status || 500);
  res.json({
    message: err.message,
    error: err
  });
});

module.exports = app;

Server code www.js:

#!/usr/bin/env node

/**
 * Module dependencies.
 */

const app = require('../app');
const http = require('http');

/**
 * Get port from environment and store in Express.
 */

const port = process.env.PORT || '4000';
app.set('port', port);

/**
 * Create HTTP server.
 */

const server = http.createServer(app);

/**
 * Listen on provided port, on all network interfaces.
 */

server.listen(port);
server.on('error', onError);
server.on('listening', onListening);

module.exports.server = server;


/**
 * Event listener for HTTP server "error" event.
 */
function onError(error) {
    if (error.syscall !== 'listen') {
        throw error;
    }

    var bind = 'Port ' + port;

    // handle specific listen errors with friendly messages
    switch (error.code) {
    case 'EACCES':
        console.error(bind + ' requires elevated privileges');
        process.exit(1);
        break;
    case 'EADDRINUSE':
        console.error(bind + ' is already in use');
        process.exit(1);
        break;
    default:
        throw error;
    }
}

/**
 * Event listener for HTTP server "listening" event.
 */
function onListening() {
    var addr = server.address();
    var bind = 'port ' + addr.port;
    console.log('Listening on ' + bind);
}

App code:

private static String url = "http://192.168.x.xxx:4000/"; //my ipv4 address 
public static final Set<String> DEFAULT = new HashSet<String>();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_beacon_results);
    Log.d("Stato:", "Sono su onCreate");
    ActionBar actionBar = getSupportActionBar();
    if (actionBar != null) {
        actionBar.setDisplayHomeAsUpEnabled(true);
        //actionBar.setTitle("Prova");
        if(getIntent().getStringExtra("BeaconHash").equals("6472324")){
            Log.d("Stato:", "Sto creando la cartella");
            actionBar.setTitle("Folder1");
        }
    }

    final View progressBar2 = findViewById(R.id.progressBar2);
    String beaconHash = getIntent().getStringExtra("BeaconHash");
    Log.d("Stato:", "URL" + " " + url + " " + "beaconHash" + " " + beaconHash);
    new AsyncHttpClient().get(url+beaconHash, new JsonHttpResponseHandler() {
        @Override
        public void onSuccess(int statusCode, Header[] headers, JSONArray responseBody) {
            Log.d("Stato:", "success");
            progressBar2.setVisibility(View.GONE);
            showResults(responseBody);
            Log.d("Risultati:", "response Body:" + responseBody);
        }
        @Override
        public void onFailure(int statusCode, Header[] headers, String responseString, Throwable throwable) {
            Log.d("Stato:", "failure");
            progressBar2.setVisibility(View.GONE);
            Toast.makeText(BeaconResultsActivity.this, "Error to load info", Toast.LENGTH_LONG).show();
            super.onFailure(statusCode, headers, responseString, throwable);
        }
    });
}