Feedback Curl error 51: Cert verify failed: UNITYTLS_X509VERIFY_FLAG_NOT_TRUSTED

Hi. I'm using Unity version 2020.1.6f1 and run on macOS Big Sur(11.2.3).

My app is a mobile application running on Android & iOS.
My app sends GET request to my client's server and get a result.
- the domain of my client's server is "https://www.sejonghakdang.org/".

It's been worked well until last month,
but I noticed that it doesn't work on real Android and iOS devices as well as the Unity editor.

error message is this:

Curl error 51: Cert verify failed: UNITYTLS_X509VERIFY_FLAG_NOT_TRUSTED

but I tested with url "http://google.com", it's working

I googled a lot of related things but got no luck.

is my client's server domain is non-secure? please let me know why this happens.

Here is my code below:

Code (CSharp):

1.     public static IEnumerator FetchGet(string uri, Action<string> action)
2.     {
3.         using (UnityWebRequest webRequest = UnityWebRequest.Get(uri))
4.         {
5.             // Request and wait for the desired page.
6.             yield return webRequest.SendWebRequest();
7.  
8.             if (webRequest.isNetworkError)
9.             {
10.                 Debug.Log("Error: " + webRequest.error);
11.                 action(null);
12.             }
13.             else
14.             {
15.                 Debug.Log("Received: " + webRequest.downloadHandler.text);
16.                 action(webRequest.downloadHandler.text);
17.             }
18.         }
19.     }

Thanks in advance.