Can not connect to domain

[Shell]

I am very new to Virtual Boxes but trying to learn.
I am trying to connect a Windows 10 Virtual Machine to a 2016 Windows Server, however when trying to connect I get an error.
Computer Name/Domain Changes
An Active Directory Domain Controller (AD DC) for the domain "xxxx" could not be contacted.
Ensure that the domain name is typed correctly.

I was wondering if someone can help me? Most of the advice I've found has been for Virtual Boxes running on Windows machines.

I've created the domain in the server, and triple-checked the IP, subnet and Primary DNS - which are all as they should be.
I'm trying to run this on an internal network, Intnet, Adapter (Desktop), promiscuous Mode (Allow VMs) - which is the same on the Virtual Box + Virtual Machines.

I have a feeling my issue is that I can't run both machines at once. So when trying to connect Windows 10 to the server, the server is powered off.

[Rootman]

First, I'd recommend using a bridged adapter not a NAT.

Next, what is the DC's OS? If it's old, like Server 2003 then it must use SMB 1 which must be installed on the Win 10 PC. If it's a Linux DC running Samba, the SMB 1 thing might be an issue there too.