LevelManager.cs

using UnityEngine;

public class LevelManager : MonoBehaviour
{
    public static LevelManager Instance { get; private set; }

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

    public void UnlockNextLevel(int currentLevelIndex, string key)
    {
        int highestUnlocked = GetHighestUnlockedLevel(key);
        if (currentLevelIndex >= highestUnlocked)
        {
            Debug.Log("Unlock next level!");
            PlayerPrefs.SetInt(key, currentLevelIndex + 1);
            PlayerPrefs.Save();
        }
    }

    public int GetHighestUnlockedLevel(string key)
    {
        return PlayerPrefs.GetInt(key, 1);
    }

    public bool IsLevelUnlocked(int levelIndex, string key)
    {
        return levelIndex <= GetHighestUnlockedLevel(key);
    }
}