Xcode - Can I develop for iOS 11.1 in El Capitan?
I hope you get a better answer, because this is not ideal but is all I have:

You could develop using the latest version of Xcode that will run on your Mac using the simulator for day-to-day testing. Then when you really need to test on a real device, upload a beta to the App Store, configure yourself as an Internal Tester and use TestFlight to download the app to your 'phone.

Edit: This may also require a full developer account!