I am having a similar issue on my site which is hosted in Azure. It's throwing a 404 when I try accessing the default sitemap route for a wordpress website: /page-sitemap.xml
I used updraft to restore my wordpress site and I also had an issue with a plugin that changed the default route of /wp-login to a custom route (for security reasons). When this plugin was installed I couldn't even access the wp-admin at all. Always would return a 404. So I went into MySql db and removed emptied out active plugins in the wp-options table. After deactivating the plugin that allowed custom login route I was able to get into the admin just fine.
Now I am noticing that my sitemap for the site is inaccessible.
Any ideas? What other details would you like?