Get the Intune enrolled devices synced into Azure Active Directory and show up as devices

When you sync your devices from ABM, it will only provision Managed Devices in Intune. 

A Managed Device in Intune does not necessarily require a Device in Azure AD (and vice versa). They are only loosely related entities. Managed Devices can exist just fine without any connection to a Device registered in Azure AD (joining would, of course, work as well, but that's not possible for iPhones) . 

If you need these devices to be available in Azure AD, you will have to register them in a separate action during your enrollment, like deploying Company Portal or Microsoft Authenticator, as Moe_Kinani already mentions.