How to install Let's Encrypt SSL certificate

Make sure your site is available in all its aliases, e.g: www.yourdomain.tld

All DNS A / AAAA records of the site and aliases must point to the server.

Then inside ISPConfig web hosting control panel perform these tasks:
  1. Go to Sites module
  2. Click on the site you want the certificate
  3. Check the Let's Encrypt SSL field
  4. Click on Save button