So I've come to the conclusion that something by default must prevent RDC from operating on a 2012 R2 DC. This particular post (link) suggests RDC on a DC is a bad idea and the I understand the reasoning.
I finally figured out I can access the DC remotely relatively easily by going through the Hyper-V application after connecting to the Hyper-V host remotely.
Once my server is up and running, I won't really have a terminal on it, so remote access would be convenient. This is a small LAN network not outward facing with very few users at present.
But, even though I will use the Hyper-V route, I would still like to know what would need to be changed from the default to allow RDC; I hoped someone might have the settings 'off the cuff'. One of the other posts I read (link) had suggestions to do this, but they looked to me to be opening far more than was necessary.
Remote Desktop Services
, so allowing normal users to RDP into the server. That's certainly not a good idea for a domain controller. But I think all you want to do is manage the server via RDP as a domain administrator, there's nothing wrong with that, just enable RDP as you would on any other Windows machine.