To get Simple Wp Sitemap working on Nginx, add the following rewrite rules for free or premium to the server { } section in your server file nginx.conf.

# Simple Wp Sitemap
location ~ /sitemap\.(xml|html) {
rewrite ^/sitemap\.(xml|html)$ /index.php?thesimplewpsitemap=$1 last;
}
# Simple Wp Sitemap Premium
location ~ /sitemap(.*)\.(xml|html) {
rewrite ^/sitemap\.(xml|html)$ /index.php?swsitemap=$1 last;
rewrite ^/sitemap-home\.xml$ /index.php?swsitemap=xml&swsitemaptype=homepage last;
rewrite ^/sitemap-(.*)\.xml$ /index.php?swsitemap=xml&swsitemaptype=$1 last;
}