AdMob ads not showing into unity game! Please help...

It's almost 2 weeks I'm trying to display AdMob 'test' ads on my app. I search every day for solution but nothing help. I'm going to share my AdManager Script and Manifest file code. If someone can help.

// AdManager.cs

 using UnityEngine;
 using GoogleMobileAds;
 using GoogleMobileAds.Api;
 using System;
 
 public class AdManager : MonoBehaviour
 {
     public static AdManager instance;
 
     private RewardBasedVideoAd rewardBasedVideo;
     private InterstitialAd interstitial;
 
     private void Awake()
     {
         if (instance == null)
             instance = this;
     }
 
     public void Start()
     {
         #if UNITY_ANDROID
         string appId = "ca-app-pub-3940256099942544~3347511713";
         #elif UNITY_IPHONE
             string appId = "ca-app-pub-3940256099942544~1458002511";
         #else
             string appId = "unexpected_platform";
         #endif
 
         // Initialize the Google Mobile Ads SDK.
         MobileAds.Initialize(appId);
 
         // Get singleton reward based video ad reference.
         this.rewardBasedVideo = RewardBasedVideoAd.Instance;
 
         // Called when an ad request has successfully loaded.
         rewardBasedVideo.OnAdLoaded += HandleRewardBasedVideoLoaded;
         // Called when an ad request failed to load.
         rewardBasedVideo.OnAdFailedToLoad += HandleRewardBasedVideoFailedToLoad;
         // Called when an ad is shown.
         rewardBasedVideo.OnAdOpening += HandleRewardBasedVideoOpened;
         // Called when the ad starts to play.
         rewardBasedVideo.OnAdStarted += HandleRewardBasedVideoStarted;
         // Called when the user should be rewarded for watching a video.
         rewardBasedVideo.OnAdRewarded += HandleRewardBasedVideoRewarded;
         // Called when the ad is closed.
         rewardBasedVideo.OnAdClosed += HandleRewardBasedVideoClosed;
         // Called when the ad click caused the user to leave the application.
         rewardBasedVideo.OnAdLeavingApplication += HandleRewardBasedVideoLeftApplication;
 
         this.RequestRewardBasedVideo();
         this.RequestInterstitial();
     }
 
     public void ShowRewardBasedAd()
     {
         if (rewardBasedVideo.IsLoaded())
         {
             rewardBasedVideo.Show();
         }
         else
         {
             MonoBehaviour.print("Dude Ads is not loaded yet");
         }
     }
 
     public void ShowInterstitialAd()
     {
         if (interstitial.IsLoaded())
         {
             interstitial.Show();
         }
         else
         {
             MonoBehaviour.print("Dude Ads is not loaded yet");
         }
     }
 
     private void RequestInterstitial()
     {
         #if UNITY_ANDROID
         string adUnitId = "ca-app-pub-3940256099942544/1033173712";
         #elif UNITY_IPHONE
         string adUnitId = "ca-app-pub-3940256099942544/4411468910";
         #else
         string adUnitId = "unexpected_platform";
         #endif
 
         // Initialize an InterstitialAd.
         this.interstitial = new InterstitialAd(adUnitId);
 
         // Called when an ad request has successfully loaded.
         this.interstitial.OnAdLoaded += HandleOnAdLoaded;
         // Called when an ad request failed to load.
         this.interstitial.OnAdFailedToLoad += HandleOnAdFailedToLoad;
         // Called when an ad is shown.
         this.interstitial.OnAdOpening += HandleOnAdOpened;
         // Called when the ad is closed.
         this.interstitial.OnAdClosed += HandleOnAdClosed;
         // Called when the ad click caused the user to leave the application.
         this.interstitial.OnAdLeavingApplication += HandleOnAdLeavingApplication;
 
         // Create an empty ad request.
         AdRequest request = new AdRequest.Builder().Build();
         // Load the interstitial with the request.
         this.interstitial.LoadAd(request);
 
 
     }
 
 
     private void RequestRewardBasedVideo()
     {
         #if UNITY_ANDROID
         string adUnitId = "ca-app-pub-3940256099942544/5224354917";
         #elif UNITY_IPHONE
             string adUnitId = "ca-app-pub-3940256099942544/1712485313";
         #else
             string adUnitId = "unexpected_platform";
         #endif
 
         // Create an empty ad request.
         AdRequest request = new AdRequest.Builder().Build();
         // Load the rewarded video ad with the request.
         this.rewardBasedVideo.LoadAd(request, adUnitId);
     }
 
     //REQUEST REWARD BASED VIDEO EVENT HANDLER
     public void HandleRewardBasedVideoLoaded(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleRewardBasedVideoLoaded event received");
     }
 
     public void HandleRewardBasedVideoFailedToLoad(object sender, AdFailedToLoadEventArgs args)
     {
         MonoBehaviour.print(
             "HandleRewardBasedVideoFailedToLoad event received with message: "
                              + args.Message);
     }
 
     public void HandleRewardBasedVideoOpened(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleRewardBasedVideoOpened event received");
     }
 
     public void HandleRewardBasedVideoStarted(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleRewardBasedVideoStarted event received");
     }
 
     public void HandleRewardBasedVideoClosed(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleRewardBasedVideoClosed event received");
     }
 
     public void HandleRewardBasedVideoRewarded(object sender, Reward args)
     {
         string type = args.Type;
         double amount = args.Amount;
         MonoBehaviour.print(
             "HandleRewardBasedVideoRewarded event received for "
                         + amount.ToString() + " " + type);
 
         Debug.Log("Type: " + type + "  Amount: " + amount);
     }
 
     public void HandleRewardBasedVideoLeftApplication(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleRewardBasedVideoLeftApplication event received");
     }
 
     //INTERSTELLER EVENT HANDLER
     public void HandleOnAdLoaded(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdLoaded event received");
     }
 
     public void HandleOnAdFailedToLoad(object sender, AdFailedToLoadEventArgs args)
     {
         MonoBehaviour.print("HandleFailedToReceiveAd event received with message: "
                             + args.Message);
     }
 
     public void HandleOnAdOpened(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdOpened event received");
     }
 
     public void HandleOnAdClosed(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdClosed event received");
     }
 
     public void HandleOnAdLeavingApplication(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdLeavingApplication event received");
     }
 }

// Manifest.xml

 <?xml version="1.0" encoding="utf-8"?>
 <!--
 This Google Mobile Ads plugin library manifest will get merged with your
 application's manifest, adding the necessary activity and permissions
 required for displaying ads.
 -->
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="com.google.unity.ads"
     android:versionName="1.0"
     android:versionCode="1">
   <uses-sdk android:minSdkVersion="14"
       android:targetSdkVersion="19" />
   <application>
   <!-- Your AdMob App ID will look similar to this sample ID:
     ca-app-pub-3940256099942544~3347511713 -->
     <meta-data
     android:name="com.google.android.gms.ads.APPLICATION_ID"
     android:value="ca-app-pub-3940256099942544~3347511713"/>
   </application>
 </manifest>

Remember I want to display test ads.