There’s something strange with uptime robots: they usually don’t detect expired SSL 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 access your site, or at least present “insecure” warnings to users.
This happens because technically, your site is not entirely down. It’s just that there’s no browser that will display your site without resulting in such warnings.
Iif you’ve generated your SSL certificate with the Let’s Encrypt Wizard in Really Simple Security, 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.