How to enable a portal when reaching a certain amount of score?

Hi! I am a new user to Unity! I am making a 3D game which consist of the player collecting 20 keys in order to make a portal to appear and end the game. When I run the game, the portal is on and when I finish collecting the 20 keys, the portal goes on and off.

All comments are appreciated!

     using UnityEngine;
     using System.Collections;
     using System.Collections.Generic;
     using UnityEngine.UI;
                     
             public Text countText;
             public Text winText;
             public int count;
             public GameObject PortalEnd;
             private bool isOn;
     
         void Start ()    
                         SetCountText();
                 winText.text = "";
                 count = 0;
                 isOn = false;
     
     
         void OnTriggerEnter(Collider other)
             {
                 if (other.gameObject.CompareTag("Keys"))
                 {
                     other.gameObject.SetActive(false);
                     count = count + 1;
                     SetCountText();
                 }
             }
     
                     
             void SetCountText()
             {
                 countText.text = "Keys obtained: " + count.ToString() + " of 20";
                 if (count >= 20)
                 {
                     winText.text = "You Win!";
                 }
             }
     
             void Update()
             {
                 if (count >= 20)
                 {
                     if (isOn == true)
                     {
                         PortalEnd.SetActive(false);
                         isOn = false;
     
                     }
                     else if (isOn == false)
                     {
                         PortalEnd.SetActive(true);
                         isOn = true;
                     }
     
                 }
     
             }