Troubleshooting Tip: Secondary device shows unseen under the Centralized Monitor (Available with Featured license)

The FortiMail detected the unseen error under the Centrally monitor even though the user mentioned the port 6688 and port 443 allowed which is needed to use Centrally Monitor communication but still the Secondary unit shows unseen. 

Primary_13 # execute telnettest 10.121.19.15:443
Connected

Entering interactive mode. Type CTRL-D to exit.

Connection status to 10.121.19.15 port 443:
Connecting to remote host succeeded.

Primary_13 # execute telnettest 10.121.19.15:6688
Connected

Entering interactive mode. Type CTRL-D to exit.

Connection status to 10.121.19.15 port 6688:
Connecting to remote host succeeded.

To correct it, enable the HTTPS service on the heartbeat interface.