On a fresh install of Windows Server 2012R2, not joined to a domain, the first network connection (obtained through DHCP) is always treated as a Public network. Normally, when joining a domain, the network connection then becomes a Domain network, but if the server will not be joined to a domain, you probably will want to set it to Private. Unfortunately, the Public Network link in Network Center is not clickable, and thus, cannot be changed. But you can make the change anyway through the Local Security Policy.

Through Powershell (preferred method):

Look up the name of the network interface (e.g. "Local Area Connection") in the network adapters list.

Open an elevated Powershell   ...