@Matt Adorjan Thanks for reaching out. When you say you are trying to access the app via internal URL : http://internalapp/ I am assuming that you are already connected to your internal corp network either directly or via some VPN.
As mobile devices which are external to corp network and are not connected to VPN would not be able to access your internal DNS and thus show the app over internal URL. If you are on corp network and still cannot access it, you would have to check for your internal DNS and see if the entries are pointing towards correct server/app.
Intune would only come to picture if everything is working before implementing the Intune app protection policies and configuration policies and when you apply Intune policies few of the urls does not open. They can help in these scenario.
For now, just verify that you are on corp network and proper entries are in place. You can match them with any other internal App URL.
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