If your sitemap is available at another URL, and not specified in your robots.txt, please provide custom sitemap(s) through your settings.
To provide a custom sitemap, go to your Dashboard, click the three dots and then click Site Settings.
Choose the tab called URL Sources.
De-select the Auto-detect setting, and then provide a list of custom sitemap URLs (one per line):
When you're done, choose Save custom sitemaps. Your next site scan will now use these sitemap settings instead.