Remote manage a non-domain server in servermanager
By default you cannot remote manage a non-domain server in servermanager, to which you are not authenticated. This happens for example when your computer is a member of a domain and the server you wish to manage is not a member of that same domain. That server is in a workgroup or a member of an other domain with no trust relationship.
To manage that computer you can add it to the trusted hosts list with a powershell command.
Set-Item wsman:/localhost\Client\TrustedHosts servername.domain.com -Concatenate -Force
The concatenate switch is used to add the server to the list of trusted hosts. If you do not add this switch the current list will be overwritten.