I have an iPhone 6s Plus connect to a MacBook Pro, although I've tried other devices as well. It shows up in Xcode and I can debug my application on the device. Also when I plug the device in it opens iTuens and shows up there as well. The problem is that it doesn't show up in the "Development Devices:" list of the Xcode screen of the Server app. As a result it doesn't show up in the devices screen when editing/creating a but either.
Configuration:
Server API v13
Xcode 8.3.1 (8E1000a), OS X 10.12.4 (16E195), Server 5.3 (16S4123)
SDKS: tvOS 14W260, iOS 14E269, macOS 16E185, watchOS 14V243
iPhone 6s Plus:
iOS: 10.3.1 (14E304)
Connected with an Apple cable!! 🙂