AbstractSingleton.cs

public abstract class Singleton<T> : MonoBehaviour where T : MonoBehaviour
{
    public static T Instance { get; private set; }

    protected void Awake()
    {
        if (Instance == null)
        {
            Instance = this as T;
            DontDestroyOnLoad(gameObject);
        }
        else
        {
            Destroy(gameObject);
        }
    }
}

LoadingManager.cs

public class LoadingManager : Singleton<LoadingManager>
{
    public void LoadNextScene()
    {
        int activeSceneIndex = SceneManager.GetActiveScene().buildIndex;
        SceneManager.LoadScene(activeSceneIndex + 1);
    }
 
    public void LoadPreviousScene()
    {
        int activeSceneIndex = SceneManager.GetActiveScene().buildIndex;
        SceneManager.LoadScene(activeSceneIndex - 1);
    }
}