If you want to test an iOS App with a enterprise certificate you have to "trust" your organization’s Apple Enterprise developer certificate, to complete the installation.
Your Testers will see the message “Untrusted Enterprise Developer” when running an app from an untrusted certificate and can trust it by following the steps outlined below on their device.
If you got this error by opening your app, go to your settings and trust the organization’s profile before you can launch the app.To resolve this, follow these steps:
Navigate to your Systemsettings > General > Device Management.
Select the developer certificate in the "ENTERPRISE APPS" section.
Press "Trust [Developer]".