iPhone showing "Untrusted Enterprise Developer" when opening Mega888? This is standard iOS behaviour for apps outside the App Store. Four steps to trust the profile.
The app installs fine, but tapping it immediately shows an "Untrusted Enterprise Developer" message and refuses to open.
This is not a sign that the app is dangerous. iOS requires manual approval for any app installed outside the App Store. Until you grant it, the system blocks the app from running.
If that did not work
If no enterprise name appears in the list at all, the install did not complete. Delete the app, reinstall the iOS profile from this portal, and do not close Safari before the install finishes. If Apple revokes the certificate (the app suddenly stops working after a few weeks), you will need to reinstall the current build — this is normal for every iOS enterprise app.