SSL Certificate installation for iOS 10.3
In iOS 10.3 and later, when you manually install a profile that contains a certificate payload, that certificate isn't automatically trusted for SSL. You must manually turn on trust for SSL when you install a profile that is sent to you via email or downloaded from a website.
If you want to turn on SSL trust for that certificate, go to Settings ⟶ General ⟶ About ⟶ Certificate Trust Settings. Under Enable full trust for root certificates, turn on trust for the certificate.
Apple recommends deploying certificates via Apple Configurator or Mobile Device Management (MDM). Certificate payloads installed with Configurator, MDM, or as part of an MDM enrollment profile are automatically trusted for SSL.