Admob banner not showing

I have created an empty on my main menu scene and have attached this code to it. However it is not showing up when i test the game on my phone. Any help would be great. Thanks

using GoogleMobileAds.Api;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class AdScript : MonoBehaviour
{

// Use this for initialization
void Start()
{

    showBannerAd();

}

private void showBannerAd()
{
    string adID = "ca-app-pub-3940256099942544/6300978111";
    /*
    //***For Testing in the Device***
    AdRequest request = new AdRequest.Builder()
   .AddTestDevice(AdRequest.TestDeviceSimulator)       // Simulator.
   .AddTestDevice("2077ef9a63d2b398840261c8221a0c9b")  // My test device.
   .Build();
    */

    //***For Production When Submit App***
    AdRequest request = new AdRequest.Builder().Build();

    BannerView bannerAd = new BannerView(adID, AdSize.SmartBanner, AdPosition.Bottom);
    bannerAd.LoadAd(request);
}

// Update is called once per frame
void Update()
{

}
 }

Have you uploaded the app?