Unable to turn the silent mode in Gionee A1 programmatically

Hey there I am using below code to turn the device into silence mode.

public static void setDeviceOnSilent(Context context) {

    AudioManager audioManager = (AudioManager) context.getSystemService(AUDIO_SERVICE);
    audioManager.setRingerMode(AudioManager.RINGER_MODE_SILENT);
}

The above code work well in all device except Gionee A1. Please suggest what is the problem i already given the NotificationAccessPolicyEnabled Permission. below is the code.

public static boolean isNotificationAccessPolicyEnabled(Context context) {
    NotificationManager notificationManager;
    notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N
            && !notificationManager.isNotificationPolicyAccessGranted()) {

        Intent intent = new Intent(
                android.provider.Settings
                        .ACTION_NOTIFICATION_POLICY_ACCESS_SETTINGS);

        context.startActivity(intent);

        return false;
    }
    return true;
}

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