iOS 12 download won’t start on Mobile data or Cellular Data plan, Apple restricted iOS download file over the air only with WiFi connection. Apple doesn’t allow downloading new iOS update while Mobile Data is turned on. But here’s the trick that works for you. A helpful solution for the Q&A: Start ‘Download iOS 12
How to download iOS 12.3.2/iOS 12 Without WiFi, With Cellular Data on iPhone, iPad, iPod Touch
