To have a Redundant setup, please enable Smart Redundancy.
Here are the steps how the feature can be enabled:
1: Login to the Zone Director.
2: Click on Configure >> System >> Check Enable Smart Redundancy.
3: Enter the Peer Device IP Address(Your Standby ZD IP Address)
4: Enter a Shared Secret(Password).
5: Login to the Secondary Zone Director follow the Step 2 to 4.
This will enable a Smart Redundancy on your Setup.
You can also refer to this article , if you have Active support: