To run a cost-per-install (CPI) campaign using the Unity Ads for iOS network, you need to implement an install tracking mechanism to register installs to your application with Unity Ads.
Unity Ads provides ready-made solutions for both client- and server-side install tracking and you should choose the one that suits you best. The recommended method is to use server-side install tracking, as it does not require an app update to be sent to the AppStore.