Hey!
What are you using in terms of the push server? Google Firebase Cloud Messaging, OneSignal or do you handle everything on your own?
If you use Google Firebase Cloud Messaging I can help you out. Basically what you need to do is to go into the Firebase Console, under Cloud Messaging choose your app for iOS, you should have already added the Apple APN certificates, or better yet, the APN key so Firebase handles that on its own.
You then need to download the GoogleService-Info.plist file and via XCode add it to your app projects Resources folder.
I’d imagine with OneSignal it is likely similar. When you use your own Push Server then you need to use the APN certificate to do some extra things. Maybe check out this Git Repo for a NodeJS based solution, should give you an idea how to do it, even if you use something else. https://github.com/node-apn/node-apn