The first one is "Settings"-"Wireless LAN"-Click on the connected WiFi-and change the DNS to 8.8.8.8.

Second, set the time to an earlier time (20 150 124 or earlier).

The third type: try "Settings"-App Store-log out of the account and log in again.

Fourth: Restore network settings (Settings-General-Restore-Restore network settings).

If none of the above methods can be used, you can call the after-sales service center for consultation. I hope my answer can help you.