I'm still stuck on iOS 7 because iTunes and the blessed updaters don't work.
iTunes will start the download, but always drops the connection before finishing. It appears to be incapable of resuming an interrupted download.
I rely on an unlimited mobile data plan from a carrier that doesn't push OTA updates, so my device will only self-update over a WiFi connection that I don't really have. Routing the connection by LTE => iPad => USB tethering to Mac => internet sharing => WiFi device-to-device network => back to iPad sort-of works occasionally. Indeed it's how I got iOS 7, but it's way too flaky now.
Is there any possibility of using a sane, stable HTTP client to get the file?