Hi P1478,
Thanks for using the Apple Support Communities.
First of all make sure your iPhone is connected to the internet.
Then Tap Settings > General > Profiles or Profiles & Device Management. Under the "Enterprise App" heading, you see a profile for the developer. You should have "Trust XXX" this option, if you can't verify/trust this app, you will need to contact the App Developer for the further assistance. Because this is the custom enterprise application (not regular public application), so the application certificate must be valid in order to trust/use this app.
Hope this is helpful.
Best Regards,
J
Install custom enterprise apps on iOS - Apple Support