So, you cannot download and install iOS 12? I tried for over 24 hours to install iOS 12 on my iPad Pro 10.5” via WiFi. It just never worked, Apple’s servers are hammered. If you don’t wish to wait a few days to install iOS 12 via WiFi, you can install it via iTunes by connecting your iPad to your Mac. You will get a bunch of “do you trust this device” stuff, then iTunes automatically says the iPad or iPhone needs to update to iOS 12. Just click through the screens and let it install. By the way, during the install my 10.5” iPad Pro received a firmware update.