HSTS and NGINX notice when your website is cached
Since Really Simple SSL pro 2.0.8 the plugin shows a notice about adding the HSTS header to the NGINX configuration when NGINX is detected as webserver. If the plugin detects NGINX, an HSTS header is inserted using PHP. While this does work in most cases it can cause issues when using caching. We therefore recommend to set the HSTS header directly in the NGINX configuration.
Depending if you want to enable preloading for your website, one of these two headers can be added to the server block in your NGINX configuration:
add_header Strict-Transport-Security: max-age=31536000
add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;
For more information about configuring HSTS and NGINX check out this article by NGINX