Exchange Management shell remotely from non-domain PC

Hello.

I need to remote Exchange management shell from a computer non-joined to the domain the exchange belongs. I am working with a windows 8.1 PC and the server is an exchange 2013CU6. I can have access through https (443). 

So far I 've tried the following:

New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUrihttps://<FQDN of Exchange Server>/Powershell/ -Authentication kerberos -Credentialdomain\username

also with UPN (username@domain.com) instead of the domain/username

in all scenarios I end up with the following error:

New-PSSession : [Exchange FQDN] Connecting to remote server <Exchange FQDN> failed with the
following error message : WinRM cannot process the request. The following error with errorcode 0x80090311 occurred
while using Kerberos authentication: There are currently no logon servers available to service the logon request.
 Possible causes are:
  -The user name or password specified are invalid.
  -Kerberos is used when no authentication method and no user name are specified.
  -Kerberos accepts domain user names, but not local user names.
  -The Service Principal Name (SPN) for the remote computer name and port does not exist.
  -The client and remote computers are in different domains and there is no trust between the two domains.
 After checking for the above issues, try the following:
  -Check the Event Viewer for events related to authentication.
  -Change the authentication method; add the destination computer to the WinRM TrustedHosts configuration setting or
use HTTPS transport.
 Note that computers in the TrustedHosts list might not be authenticated.
   -For more information about WinRM configuration, run the following command: winrm help config. For more
information, see the about_Remote_Troubleshooting Help topic.
At line:1 char:1

I have normal access through ecp. Any ideas will be appreciated.