How are you building your app? What command(s)/tool(s) are you using to build your app?
It's an angular 2 application. I am using web pack to transpile and build the application in a dist folder that I then upload to the Adobe Build website to build the PhoneGap app itself. I then download the build, upload it to iTunes Connect, issue it as a build in TestFlight, and download it to the iPhone from TestFlight.
Alternately, I've also tried just locally running the PhoneGap app on my Mac from the dist folder itself, using the PhoneGap app on the iPhone to run the app. Still was not able to connect to debug. Hope that helps.
What kind of certificate have you provided to PhoneGap Build? See the docs for more: Signing | PhoneGap Docs
Inspectable apps must be signed by your developer certificate, NOT a distribution certificate. As such, this rules out using the PhoneGap Developer App (since it is distributed by the App Store) unless you build a version yourself. Double check how you're signing the app with PGB as well to make sure you're using a developer cert and not a distribution cert.
You might also just want to set up a local PhoneGap or Cordova CLI instance and build locally.