Identify unity advertisement is finished from another script?

When I'm trying to get to know whether my advertisement is finished or not, It doesn't work properly. In the Level1 script, it directly runs without waiting.

I have several levels let say level 1,2 3 and one AD display script.

Sample Level1 Script

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 
 
 public class Level1 : MonoBehaviour
 {
     public GameObject slot1;
     public GameObject slot2;
     public GameObject slot3;
 
     public GameObject Chemical1;
     public GameObject Chemical2;
     public GameObject Chemical3;
 
     public int[] moves = new int[2];
     private static int moveCount =0;
     public static int starCount =0;
     private static bool answer;
 
     ScoreManager scoreManager = ScoreManager.Instance();
     // Start is called before the first frame update
     void Start()
     {
         
         moves[0] = 3;
         moves[1] = 5;        
     }
 
     // Update is called once per frame
     void Update()
     {
         
     }
 
     public void onCheckButtonClick()
     {
         moveCount = scoreManager.getMoveCount();
         scoreManager.setMoveCount();
         int currentLevel = SceneManager.GetActiveScene().buildIndex -1;
 
         if ((slot1.transform.position == Chemical1.transform.position) &&
             (slot2.transform.position == Chemical2.transform.position) &&
             (slot3.transform.position == Chemical3.transform.position)
             )
         {
             Debug.Log("Hell yeahh, you correct mann......!");
             if(moveCount <= moves[0])
             {
                 starCount = 3;
             }
             else if (moveCount <= moves[1])
             {
                 starCount = 2;
             }
             else
             {
                 starCount = 1;
             }
 
             PlayerPrefs.SetInt("starCount", starCount);
             PlayerPrefs.SetInt("currentLevel", currentLevel);
             PlayerPrefs.SetInt("answer", 1);
 
             SceneManager.LoadScene(1);
 
         }
         else
         {
             PlayerPrefs.SetInt("starCount", 0);
             PlayerPrefs.SetInt("currentLevel", currentLevel);
             PlayerPrefs.SetInt("answer", 0);
 
             SceneManager.LoadScene(1);
         }
         
     }
 
     public void onHomeButtonClick()
     {
         SceneManager.LoadScene(0);
     }
 
     public void onHintButtonClicked()
     {
         //ask dialog box
         //implement advertiestment here
         FindObjectOfType<ShowAd>().DisplayReweredVideo();
         if (FindObjectOfType<ShowAd>().rewardPlayer())
         {
             giveHint();
         }
         Debug.Log("Hint button clicked");
     }
 
     public void giveHint()
     {
         Chemical1.transform.position = Vector3.Lerp(Chemical1.transform.position, slot1.transform.position, 1);
     }
 }
 

Show AD script

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Advertisements;
 
 public class ShowAd : MonoBehaviour, IUnityAdsListener
 {
     // Start is called before the first frame update
 
     private string googlePlayID = "3980107";
     bool gameMode = true;
     string myPlacementId = "rewardedVideo";
 
     private bool eligibleForReward;
     void Start()
     {
         Advertisement.AddListener(this);
         Advertisement.Initialize(googlePlayID, gameMode);
     }
 
     public bool rewardPlayer()
     {
         return eligibleForReward;
     }
 
     // Update is called once per frame
     public void DisplayReweredVideo()
     {
         Advertisement.Show(myPlacementId);
     }
 
     // Implement IUnityAdsListener interface methods:
     public void OnUnityAdsDidFinish(string placementId, ShowResult showResult)
     {
         // Define conditional logic for each ad completion status:
         if (showResult == ShowResult.Finished)
         {
             eligibleForReward = true;
             // Reward the user for watching the ad to completion.
         }
         else if (showResult == ShowResult.Skipped)
         {
             eligibleForReward = false;
             // Do not reward the user for skipping the ad.
         }
         else if (showResult == ShowResult.Failed)
         {
             Debug.LogWarning("The ad did not finish due to an error.");
         }
     }
 
     public void OnUnityAdsReady(string placementId)
     {
         // If the ready Placement is rewarded, show the ad:
         if (placementId == myPlacementId)
         {
             // Optional actions to take when the placement becomes ready(For example, enable the rewarded ads button)
         }
     }
 
     public void OnUnityAdsDidError(string message)
     {
         // Log the error.
     }
 
     public void OnUnityAdsDidStart(string placementId)
     {
         // Optional actions to take when the end-users triggers an ad.
     }
 
     // When the object that subscribes to ad events is destroyed, remove the listener:
     public void OnDestroy()
     {
         Advertisement.RemoveListener(this);
     }
 }
 

What is the proper way to trigger the giveHint() function in Level1 script after playing advertisement?

(Please note that there are several scripts like level1)

Can anyone guide me to solve this issue? Thanks in advance.