With Apps option removed from latest iTunes 12.7.... , how to install adhoc IPA?
You can still install ad-hoc apps using iTunes by following the steps under Manually add items from your computer in this tech note
The steps are:
Apple Configurator 2 software from Apple can be used for sideloading - https://itunes.apple.com/us/app/apple-configurator-2/id1037126344?mt=12
Just install the software, connect your device, drag your ipa into the Configurator, as simple as that.