When a computer is not a domain member and there are no SSL Certificates available for HTTPS. You must change the trusted hosts on the client side.
Then: Computer Configuration -> Administrative Templates -> Windows Components -> Windows Remote Management (WinRM) -> WinRM Client
Last: Enable “Trust the remote machine” and add the server to the trustedhost field.
Or with PowerShell:
Set-Item -Path WSMan:localhostClientTrustedHosts -Value nanoIPv4address