Hello,
Thank you for contacting us and sorry for any inconvenience that might have been caused due to that.
Please note that some options are only available when you enable the Advanced Mode in Rank Math. If you’re unable to find the 404 Monitor Module in Rank Math Dashboard, you must have chosen to use the Easy Mode. Please try to switch in Advance mode from Rank Math > Dashboard.

That should reveal the module on Rank Math Dashboard.
You can follow this URL for more details on Advanced Mode:
https://rankmath.com/kb/advanced-mode/
Hope that helps and please don’t hesitate to let us know if you have any other questions.
Thank you.
Hello,
We are super happy that this resolved your issue. If you have any other questions in the future, know that we are here to help you.
If you don’t mind me asking, could you please leave us a review (if you haven’t already) on https://wordpress.org/support/plugin/seo-by-rank-math/reviews/#new-post about your overall experience with Rank Math? We appreciate your time and patience.
If you do have another question in the future, please feel free to create a new forum topic, and it will be our pleasure to assist you again.
Thank you.