When trying to install an app from an unknown developer on iOS 9, 10, and 11, you will see an alert dialog that the developer is not trusted similar to the following screendshot.
You can solve this as follows:
Open Settings.app.
Go to General > Profiles (or General > Device Management, or General > Profiles & Device Management)
Tap on the row with the developer's name.
Tap
Trust "<name>".
After this, you should be able to launch the app.