Why does random selection of an element from an array not work?

Question

C++ / C#

Here is the code itself, it seems to include a random object, but for some reason, everything in turn

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class LevelPlay : MonoBehaviour
{
    public bool isDead = false;
    public GameObject[] OnButtons;
    public bool IsStart = false;

    IEnumerator LevelStart()
    {
        while (isDead == false)
        {
            yield return new WaitForSeconds(0.5f);
            int rnd = Random.Range(0, 215);
            OnButtons[rnd].SetActive(true);
        }
    }

    private void Start()
    {
        StartCoroutine(LevelStart());
    }
}

Answer 1

var random = new Random(DateTime.Now.Millisecond);
int rnd = random.Next(OnButton.Length);