How to randomly instantiate an object at specific locations?

I have a room where I am trying to randomly instantiate portals on the walls of the room at specific points.

So far, I have been able to destroy and randomly instantiate portal prefabs but it only works once when I first run the application. I have two scripts in which ABChecker is used to detect two collisions against a cube and then it calls the Reset() sub-routine in the other script. And then shrinkRoom is used to destroy the current portal prefab and instantiate another portal randomly onto a specific point.

One thing to note is that there are two portals in the room, directly opposite to each other. Currently the main problem that I am facing is that SelectPortals() in shrinkRoom only randomizes the location of instantiation once.

Thanks in advance.

The scripts are as follows: shrinkRoom

 public class shrinkRoom : MonoBehaviour {
     public float countDown;// this is a timer for each room
     public static int PortalNum;
 
     public GameObject player;
 
     //the booleans are to check which walls need to be shrinked
     public bool ABshrink;  
    
     //these are other scripts that get activated when the booleans are pressed
     public ABChecker ABChecker_portalA;
     public ABChecker ABChecker_portalB;
 
     public GameObject Portal; // portal gameobject that gets spawned
     public GameObject[] SpawnedPortal; // contains all portals 
 
     public GameObject[] A_portalPos; // array containing six possible portal positions for wall A
     public GameObject[] B_portalPos; // array containing six possible portal positions for wall B
 
     int size;
 
     float roomX;
     float roomY;
     float shrinkSpeed;
     float playerSize;
     //this float is just the rate of the room getting shrinked
     void Awake() {
         roomX = transform.localScale.x;
         roomY = transform.localScale.y;
 
         //picked X scale because its the same and Y could also be used with same effect (room is a cube)
         shrinkSpeed = (roomX - 1 / countDown) * Time.deltaTime;
         size = A_portalPos.Length;
         SpawnedPortal = new GameObject[size];
         PortalNum = Random.Range (0, size);
         SelectPortals();
     }
 
     void Update() {
 
         if (countDown > 0) {                
                          //as long as the timer is not 
                          //time.detaTime records the time taken for one frame.complete...
             countDown -= Time.deltaTime;    //it needs to deduct one second from the timer
 
             //clamps the variable so that negative scaling (which would result in room increasing in size) is not possible
             // 1 is the minimum possible value and the one that we need  NOTE: Must be changed to player width using bounds of camera box collider
             //8 is the maximum possible value that is not needed so 8 is just a random number to suit the parameters
 
             if (ABshrink == true) {
                 ABChecker_portalA.enabled = true;
                 ABChecker_portalB.enabled = true;
             }
 
             //this makes it scale down as long as the current scale co ordinates are still in the range of the clamp values
 
             float scaleX;  //stored the scale of the gameObject's x  co-ordinate in a variable
             float scaleY;  //stored the scale of the gameObject's y  co-ordinate in a variable
             scaleX = Mathf.Clamp(transform.localScale.x, 1, 8);
             scaleY = Mathf.Clamp(transform.localScale.y, 1, 8);
 
             if (transform.localScale.x == scaleX && ABshrink == true) {
                 transform.localScale -= new Vector3(shrinkSpeed, 0, 0);
             }
         }
     }
 
     public void Reset() {
         Debug.Log("Reset");
         player = GameObject.Find("Cube");
         transform.position = player.transform.position;
         transform.localScale = new Vector3(roomX, roomY, transform.localScale.z);
         Destroy(SpawnedPortal[0]);
         Destroy(SpawnedPortal[1]);
         SelectPortals();
     }
 
     public void SelectPortals() {
         Debug.Log("Select Portals");
 
         //PortalNum = Random.Range(0, size );
         Debug.Log ("Random no. is " + PortalNum);
 
         GameObject[] PortalChoosenAB; // these contain the correct portals
         GameObject wallA;
         GameObject wallB;
 
         wallA = GameObject.FindGameObjectWithTag("WallA");
         wallB = GameObject.FindGameObjectWithTag("WallB");
 
         if (ABshrink == true) {
             Debug.Log("PORTAL NUM" + PortalNum);
             PortalChoosenAB = new GameObject[2];
 
             PortalChoosenAB[0] = A_portalPos[PortalNum];
             PortalChoosenAB[1] = B_portalPos[PortalNum];
 
             SpawnedPortal[0] = Instantiate(Portal, PortalChoosenAB[0].transform.position, Quaternion.identity);
             SpawnedPortal[0].transform.parent = wallA.transform;
 
             SpawnedPortal[1] = Instantiate(Portal, PortalChoosenAB[1].transform.position, Quaternion.identity);
             SpawnedPortal[1].transform.parent = wallB.transform;
 
             ABChecker_portalA = SpawnedPortal[0].AddComponent<ABChecker>() as ABChecker;
             ABChecker_portalB = SpawnedPortal[1].AddComponent<ABChecker>() as ABChecker;
             PortalNum = Random.Range (0, size);
         }
     }
 }

And ABChecker :

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 // summary of script: to make sure both shrinking walls are colliding and only then remove the room
 
 public class ABChecker : MonoBehaviour {
     public static int hits;//this counts the hits it makes with the walls
     public GameObject newRoom;
     public shrinkRoom scriptShrinkRoom;
 
     void Awake() {
         newRoom = GameObject.Find("newRoom");
         scriptShrinkRoom = newRoom.AddComponent < shrinkRoom >() as shrinkRoom;
     }
 
     void OnCollisionEnter(Collision col) { //this function is called the first frame a collision is detected
     
         if (col.gameObject.name == "Cube"){//if the gameobject that is being collided with has the name "Cube"
             hits++; // add one to hits
             Debug.Log(hits); //testing
             Debug.Log(col.gameObject.name);
         }
     }
 
     void OnCollisionExit(Collision col) {// this is called the frame when it no longer collides with a game object
         hits--; // it deducts from the number of DIFFERENT collisions with walls as it no is not colliding with a wall
     }
 
     void Update() {
         if (hits == 2) { //so only when BOTH walls A and B are colliding with the gameobject it will create a new room
             scriptShrinkRoom.Reset();
         }
     }
 }