Just to confirm, your controllers are ZD and not SZ or vSZ correct? If the controllers are Zone Directors, you will have to first create the Wireless Cluster in RDD. For the "System ID", you can pick any meaningful name (e.g. ZD-MQTT).
Once the Wireless Cluster is added, go ahead and create a Wireless Controller in RDD for each Controller you have. Make sure to set the Cluster for the Controller to the Wireless Cluster you created above. For the IP, use the IP for each Controller that you can SSH into as RDD will need to be able to SSH into the ZD controller to be able to pull the data/stats it needs.
Add-on to what Xang suggested, please note below points related to ZD running in smart redundancy.
You can add ZD cluster (two ZDs in smart redundancy mode) using single management IP address (For this you have to make sure management interface is enabled on ZD and configured with an IP address).
If you add both the ZDs to RDD, then RDD will see them as two different devices, means comparing data for both may not be the same because only one of the ZDs will be active at a time for AP operations and management.
As a best practice, its better to use management IP or If you want the monitoring of both the devices, then add both ZDs.