Issue
I need to have an app in a Private Space send requests to another app in the same Private Space. When the app code tries to connect to the other app, the connection time outs.
Resolution
If your app doesn't have to be publicly available, consider using Internal Routing.
Alternatively, add the Stable Outbound IP addresses of the Private Space to the list of Trusted IP ranges.
Even if non-internally-routed apps are in the same Private Space, requests to an app through the custom domain (or the default *.herokuapp.com domain) goes through the routing layer of the Private Space where the Trusted IP ranges are enforced.