I found this to be the problem I had:
"If the server is a domain controller, do you also use it for DNS because the
DNS server IP address points to the router? Change it to the DC/DNS server.
Active Directory relies on DNS, and you should run DNS on the server, not on
the router. To get Internet access, configure the FORWARDER to the ISP's DNS
server under the DNS server properties in the DNS management console."
From: A Domain Controller for the Domain XXX Could Not be Contacted
My lab desktop NIC was getting its DNS information from the router. I left the router to handle DHCP, but I manually entered the DNS IP address of my domain controller (which is also my DNS server). Since the domain controller has a forward to my router, I can still get Internet access.
With this done, the lab desktop was added to the domain without a problem.