If you use your own server certificates that are not commonly trusted, you have to add the corresponding CA as a trusted root certificate on the clients.
For detailed information about adding trusted root certificates consult the documentation of the corresponding operating system.
Microsoft Windows
You can use the “Certificates” MMC snap-in.
Apple macOS
Double-click the CA certificate, add it to the keychain and choose “Always Trust”.
Ubuntu
Store the CA certificate (X.509 format with crt as extension) in /usr/local/share/ca-certificates/ and execute the following command: sudo dpkg-reconfigure ca-certificates