A wrong step is mentioned in the HA configuration steps.

https://www.mongodb.com/docs/ops-manager/current/tutorial/configure-application-high-availability/

Step 4 says:

Change the Ops Manager URL to the Load Balancer URL in the MongoDB Agent configuration file.

Complete the following steps on each MongoDB Agent's host.

1. Edit the MongoDB Agent configuration file.
   vi /path/to/configurationFile.config
   The location of the Automation configuration file depends on your platform:

   /etc/mongodb-mms/automation-agent.config

1. Change the mmsBaseUrl property to point to the load balancer.
   mmsBaseUrl=<LOAD-BALANCER-URL>:<PORT>
2. Click Downloads & Settings.

1. Click Save & Close.

1. Restart the MongoDB Agent.

Steps 3 and 4 don't make much sense. Just changing the config file and restarting the service should be enough.