FortiMail 5.4 HA Link Monitor Configuration

I'm newly working with the FortiMail system, and I'm having a hard time setting up the HA cluster. The section about link monitoring isn't very clear - they only mention what it's for, rather than which ports need to be set up and how.

First, I set up the network similar to what was in the example in the guide (please see the attached file image), except I used the IP addresses for my network. However, when I killed the primary device, the secondary device failed to take over the master role. Do I also need to set up the link monitor separately under sysem>network>link monitor? Or is there another step that I'm missing? The cluster seemed to work well when I just restarted the primary device, because the secondary device's status went to "hold-off" due to having received the waiting command from the primary device. However, when I killed port 1, nothing happened.

Does anyone know what went wrong?

Thanks in advance for your help.