Update iPhones over the air with private server
You can run the Caching service, part of server.app. The Caching service supports OTA updates for iOS 7 and later. All you need to do is enable the service in Server.app. Devices do not need to be part of MDM supervision.


  macOS and iOS devices automatically access the available caching service on the network they’re currently connected to


You can find Apple's documentation of the Caching service at:


https://help.apple.com/serverapp/mac/#/apd74DDE89F-08D2-4E0A-A5CD-155E345EFB83