Network Location Access Control - How to Look Up Blocked Attempts

We're having an issue with an outside vendor getting blocked when attempting to open SharePoint docs.  We use Network Location Access Control (IP whitelisting) but cannot determine what new IPs to allow.

We've tried looking in the audit logs in Security Center but that only shows failed (or successful) authentication attempts.  Is there a log that shows what IPs have been blocked when trying to access SharePoint?