Help to understand events and delegates (Game Manager)

Hi everyone, I'm new to Unity so I have a lot to learn..

I'm trying to set up a game manager for my game, I have been following this tutorial and understood most of it: https://hub.packtpub.com/creating-simple-gamemanager-using-unity3d/

Now, as per the tutorial, I have a game manager script that is a Singleton, and I have a scene called Intro, that has an empty gameobject with an Intro.cs script.

The issue I have is that when I run the game, the Intro script executes Awake and Start, but the ChangeState function is not either called, or passed to the delegate (not sure about the correct wording). I cannot really tell where the problem is, and I hope you guys can help me understanding it.

GameManager.cs

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 //list of game states
 public enum GameState { INTRO, MAIN_MENU }
 
 //callback to change the gameState
 public delegate void OnStateChangeHandler(); 
 
 public class GameManager
 {
     protected GameManager() {}
     static GameManager instance = null; //global variable where the gm instance is stored
     public event OnStateChangeHandler OnStateChange; //event to change state
     public GameState gameState { get; private set; } //getter for the current game state
 
     //getter for the gm instance
     public static GameManager Instance{
         get {
             if (GameManager.instance == null) { //if there is no instance, create one
                 GameManager.instance = new GameManager();
             }
 
             return GameManager.instance; //returns the instance
         }
     }
 
     //function to change the state
     public void SetGameState(GameState state) {
         this.gameState = state;
         OnStateChange();
     }
 
     public void OnApplicationQuit() {
         GameManager.instance = null;
     }
 }

Intro.cs

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 
 public class Intro : MonoBehaviour
 {
     GameManager gm;

     void Awake() {
         gm = GameManager.Instance;
         gm.OnStateChange += ChangeState;
         Debug.Log("Awake - current state: " + gm.gameState);
     }
 
     void Start() {
         Debug.Log("Start - current state: " + gm.gameState);
     }
 
     public void ChangeState() {
         gm.SetGameState(GameState.MAIN_MENU);
         Debug.Log("About to change state to: " + gm.gameState);
         Invoke("LoadLevel", 2f);
     }
 
     public void LoadLevel() {
         SceneManager.LoadScene(1);
     }
 
 }

Thanks for your support!