Long story short, I had a DC running perfectly but it was on Server 2012 R2 Standard Evaluation. In order to change from Eval. to Full, I had to create a 2nd DC server, transfer Operational Masters, demote the original DC server, remove ADDS, convert the license to Full, activate, reinstall ADDS, Promote Server and transfer FSMO's back.
Everything has worked without issue up to re-promoting the DC server...
When I try I get the error
An Active Directory domain controller for the domain "contoso" could not be contacted. Ensure you have the correct DNS domain name.
I'm using the correct credentials, domain name etc. yet the same procedure I followed to add the 2nd DC server earlier on is now not working for adding the original DC back. I've even gone as far as to remove the DNS roles from the server and point the Primary DNS to the temporary DC server, but this hasn't helped.
What do I do?