Thank you for contacting Rank Math support.
The Instant Indexing module of our plugin is for IndexNow API, which submits the URLs to Bing and Yandex. For Google, you will have to use the Instant Indexing plugin. Here is a guide to help you set up the Instant Indexing plugin: https://rankmath.com/blog/google-indexing-api/
It is still recommended that you submit your sitemap to Google Search Console.
Hope that helps and please do not hesitate to let us know if you need our assistance with anything else.