You have an Azure Active Directory (Azure AD) tenant that syncs to an Active Directory forest. The tenant-uses through authentication.
A corporate security policy states the following:
Domain controllers must never communicate directly to the internet.
Only required software must be- installed on servers.
The Active Directory domain contains the on-premises servers shown in the following table.

You need to ensure that users can authenticate to Azure AD if a server fails.
On which server should you install an additional pass-through authentication agent?