try to change ur dns to google public dns or open dns instead, from personal experience they are usually less problematic, especially with iOS Appstore. However, "most of the time" ur ISP's default DNS will provide better experience, i.e. better CDN routing, better speed. Btw, remember to restart the device after you change the DNS.
Change DNS Settings

This post has been edited by stanleyhks: Nov 22 2017, 02:08 PM