When connected to the same Local Area Network (LAN) as your server, you can use its .local URLs for fast and secure connections.
Ensure you have properly downloaded and trusted your Root CA, including configuring Firefox if you are using it (recommended).
Ensure your client device (phone/laptop) is connected to the same Local Area Network (LAN) as your server. This usually means your server and your client device are using the same router, either by ethernet or WiFi
On Windows, it is necessary to install Bonjour Print Services in order to access the .local URLs of your installed services. In a future release of StartOS, this will no longer be necessary.
Simply install Bonjour Print Services from Apple: https://support.apple.com/kb/DL999
Warning
Bonjour can be unreliable. If your local URLs suddenly stop working, you may need to uninstall and re-install Bonjour. Go to System Settings > Remove Programs, uninstall Bonjour and Bonjour Print Services, re-install Bonjour Print Services from the link above, then restart Windows.