Hello,
Thank you for contacting Rank Math and bringing your concern to our attention.
Yes, you can use and set up Rank Math on a staging/subdomain site.
There is no unique way of treating a staging site that is different than the production site. Once you have pushed your site to live, all the Rank Math settings you have made or imported to your staging site would still be intact.
The basic difference would be the indexing option, which you can easily set using the WordPress option to discourage search engines from indexing your site in WordPress Dashboard > Settings > Reading.

Other than this, you should treat the settings on your staging site the same as you would on your live sites.
You won’t however need to enable some modules such as Sitemap, 404 Monitor & Analytics on the staging area since they won’t be useful.
Lastly, after pushing your site to production/live, you’ll need to reconnect your website to your Rank Math account to replace the subdomain in the list of websites.
Hope that helps.
Thank you.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.