Countdown clock to persist over scenes

Hi,

I am new to unity and I have two problems with the Countdown Clock: Problem one is that I want to have a Countdown Clock that persists over 3 Scenes. Right now it resets in every new Scene. My second Problem is that I need the Countdown Clock to start in the second scene but should keep running when i reenter the first Scene. The 3 Scenes are rooms that are connected with each other.

The script that I have so far is this:

 using System.Collections;
 using System.Collections.Generic;
 using System.Globalization;
 using System.Threading;
 using UnityEngine;
 using UnityEngine.UI;
 using UnityEngine.SceneManagement;
 
 
 
 public class CountdownClock : MonoBehaviour
 {
    [SerializeField] private Text uiText;
    [SerializeField] private float mainTimer;
 
     private float timer;
     private bool canCount = true;
     private bool doOnce = false;
 
       
    
     void Start()
     {
     timer = mainTimer;
      
 
     }
 
     void Update()
     {
      if (timer >= 0.0f && canCount)
     {
         timer -= Time.deltaTime;
          //uiText.text = timer.ToString("F");
 
 
             string minutes = Mathf.Floor(timer / 60).ToString("00");
             string seconds = (timer % 60).ToString("00");
 
            uiText.text = (string.Format("{0}:{1}", minutes, seconds));
         }
 
      else if (timer <= 0.0f && !doOnce)
     {
         canCount = false;
         doOnce = true;
         uiText.text = "00.00";
         timer = 0.0f;
             GameOver();
         }
 
     }
    
     void GameOver()
     {
         //load a new scene
         SceneManager.LoadScene("GameOverScreen");
     }
 }
 

Hopefully someone can help me with this! :)