PSRemoting fails through VPN

My laptop is domain joined and I use PSRemoting just fine from my office. I get an error when trying to connect outside the office via VPN. I have triple checked our firewall but the issue seems to be with the client or server since "Test-WSMan" succeeds with no issues. I have added the machines to each others' trust hosts and try many other things with no success. Please help!

I am a member of Domain Admins and the laptop is on the domain.

Enter-PSSession : Connecting to remote server SERVERNAME failed with the following error message : WinRM cannot
process the request. The following error with errorcode 0x80090311 occurred while using Kerberos authentication: The
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
+ Enter-PSSession SERVERNAME
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidArgument: (SERVERNAME:String) [Enter-PSSession], PSRemotingTransportException
    + FullyQualifiedErrorId : CreateRemoteRunspaceFailed

From the issue, my understandings are as follows:

• While adding the trusted hosts, you should add the servers with FQDN like

Set-Item wsman:\localhost\Client\TrustedHosts -Value Server01.Domain01.Fabrikam.com

• During Enter-PSSession, I would recommend you not to use the Kerberos as -Authentication, you can use default since its a domain admin account.
• I believe the ports are open from the network side since you have already done that. But make sure to cross-check the state of the port; Its listening properly or not.

I believe these points will help you. Normally, VPN doesnot give this much issue unless the hops are really complicated.