Unity Ads not working on my Android device but works on many other devices

I have a Sony Xperia Z5 Premium with android 6.0.1. When I try to call Unity Ads, nothing shows up. On every other devices like Samsung S3 , S6 , Note 2 , Note 4 and two tablets , it works.

Another apps which also have Unity Ads are working well. Even our previous product works well on my device. Every other projects show Unity Ads on my device.

I am calling the ad at the very beginning of the game.

     void Start() {
             Advertisement.Initialize("1234567",true);
             StartCoroutine(ShowAdWhenReady());
    }
     
    IEnumerator ShowAdWhenReady() {
             while (!Advertisement.IsReady())
                     Advertisement.Show();
    }

In this case, Advertisement.IsReady() never becomes true on my device. However, I got this log on Log Viewer: HandleInterstitialLoaded event received.

I have done everything listed below:

1. Creating another empty project and exporting the old one there

2. Changing Unity Version (I have tried 5.3 , 5.4 and 5.5)

3. Restarting the phone

4. Deleting existing APK from the device and fresh install

5. Trying rewardedVideo and video ads

6. Trying Test Ad

Nothing works. I hope someone know the issue and the solution.

Best regards Ozan