The issue described pertains to iOS devices not checking into Workspace ONE and push notifications not being sent. This is typically related to the Apple Push Notification Service (APNs) certificate, as iOS devices rely on APNs for communication. Therefore, the administrator should:
C. Make sure that the APNs certificate is valid and not expired.
The APNs certificate is crucial for iOS device communication with Workspace ONE, and if it's invalid or has expired, it can result in the described issues. Ensuring that the APNs certificate is valid and up-to-date should help resolve the problem with iOS devices not checking in or receiving push notifications.