If you manually install an app on iOS 9+, you must also manually establish trust.
Follow the steps to install: Settings.app -> General -> Profile -> Select Profile -> Trust.
If you manually install an Adhoc app, make sure uninstall the same app from App Store!!
iOS signature certificate issues
In iOS, Apple has designed a signature verification mechanism for apps developed, tested,
and shelved in order to prevent apps from being distributed in the App Store app store.
The signature certificate is valid for one year from the date of issuance. After the expiration,
you need to re-generate and re-sign the app before you can continue to install it.
Tell the developer to go to the Apple developer website to regenerate the signature certificate, build app and upload it.
Regenerating a signed certificate will not have any effect on the previous application, i. e. once expired it will always be expired, unless re-sign the app.