Hello,
Thank you for your query and we are so sorry about the trouble this must have caused.
I checked your sitemap and it seems to be loading just fine on my browser.
I went ahead and check all the variations of your sitemap using https://httpstatus.io/ and we saw two Status codes. Please refer to my screenshot here:

Only one of the 4 variations should show a 200 status code. Three (3) of the sitemaps should show 301.
Please follow the steps in this link:
https://rankmath.com/kb/couldnt-fetch-error-google-search-console/#5-check-for-the-correct-version-of-the-sitemap and redirect to the correct version.
Then, please ensure that your sitemap is not heavily cached as well. Please follow the steps below to address it:
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/
Once done, resubmit the sitemap_index.xml in the Search Console and wait for Google to recrawl the sitemap.
Let us know how that goes. Looking forward to helping you.