The steps to ensure that the Monitoring Console (MC) is aware of the additional indexers are to use the MC setup UI, review and apply the changes. The MC setup UI is a graphical interface that allows you to configure and manage the MC instance and its data sources. When new indexers are added to an indexer cluster, the MC setup UI can detect them automatically and prompt you to review and apply the changes. This will update the MC data sources and enable the MC to monitor the new indexers. References:

https://docs.splunk.com/Documentation/Splunk/9.1.1/DMC/ConfiguretheMonitoringConsole

https://docs.splunk.com/Documentation/Splunk/9.1.1/DMC/Addinstances