Display.displays.Length is count "2". But multi monitor doesn't work! :(

Hi, I have some problem.

First, my computer environment is below:

1. OS : MacOS (Macbook pro 2015)

2. Unity3D version : 2019.1.6f1 Personal

3. Secondary monitor: LG Monitor (This is connected HDMI from Macbook)

Second, I setting up unity project below:

1. Display Resolution Dialog : Enabled

Third, I had wrote some code below:

 using UnityEngine;
 using UnityEngine.UI;
 
 public class Dual : MonoBehaviour {
      public Camera camera1, camera2;
      public Text myText;
 
      void Start() {
          for (int i = 0; i < Display.displays.Length; i++) {
                 Display.displays[i].Activate(Display.displays[i].systemWidth, 
                       Display.displays[i].systemHeight, 60);
          }
 
          myText.text = (Display.displays.Length).ToString();
          Debug.Log("Displays connected: " + Display.displays.Length);
      }
 }

So, I had build and run in my computer. "Display.displays.Length" text value is "2".

My macbook monitor is shown, but my secondary monitor (LG Monitor) is not show anything. I don't know.

How can I this problem solve..?

I'd really appreciate it if you let me know.

Regards.