There’s something strange with uptime robots: they don’t detect expired certificates. So, even though you don’t get any messages that your site is down, it could still be that your SSL certificate has expired… and browsers will block your site, or present “insecure” warnings to users.
This happens because technically, your site is not down. It’s just that there’s no browser that will display your site without resulting in such warnings.
If you generate your SSL certificate with the Let’s Encrypt Wizard in Really Simple SSL, the plugin will send you an email when your certificate is about to expire within two weeks.
This way, you will be notified by email when your Let’s Encrypt SSL certificate is expiring soon, so you can take appropriate action by generating a new, valid SSL certificate.
