Hello,
Thank you for contacting Rank Math support, and sorry for any inconvenience that might have been caused due to that.
I checked your sitemap page in a sitemap validator and it seems to be validating as fine as you can see in my screenshot below:

Also, the robots.txt file is also configured correctly:

Going back to your sitemap, can you please follow these steps?
1. Flush the Sitemap cache by following this video screencast:
https://i.rankmath.com/pipRDp
2. Exclude the Sitemap files of the Rank Math plugin in your caching plugin. The cache could be via a plugin or from the server. For plugins or Cloudflare, please follow this article:
https://rankmath.com/kb/exclude-sitemaps-from-caching/
3. After that, remove all your sitemaps from your GSC and resubmit only the primary sitemap (sitemap_index.xml).
Please note that the “Discovered URLs 0” that you see in Google Search Console just means that the URLs in the sitemap were already found by Googlebot by other means, instead of via the sitemap. For example, Googlebot may have found the URLs from internal links or possibly from links on another site.
The crawling and indexing of the website depend on the authority and crawl budget of the website.
Lastly, with regard to your traffic issues, could you please check the coverage report under your Google Search Console account to see if there are any errors shown?
Looking forward to helping you on this one.
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.