The certificate for HiveManager NG has expired on October 23, 2019. As a result, Dell N Series switches running an older firmware or an older version of the HiveAgent, can no longer connect or be connected to HiveManager NG after this date.
The solution is to either update the switch firmware to at least 18.104.22.168, or manually update the HiveAgent to 1.1.29.
More information: https://www.dell.com/support/article/en-us/sln319098/dell-emc-networking-hivemanager-site-cloud-aerohive-com-certificate-expiration-how-to-manually-upgrade-hiveagent-on-dell-n-series-switches?lang=en&ref=topsolutions
A very good guide on how to configure your Aerohive wireless network to use RADIUS authentication to authenticate users against a Windows Active Directory, and setting network access (VLAN) based on their group membership: