Long shot, but make sure the "DNS Client" service is set to Automatic and is started on the PC. Also, as Throrik says, make sure the primary DNS of the network card is set to the IP of your "server".
I always do the manual domain join via "This PC, properties, etc.". Make sure your PCs are running the Pro version.
I can't remember the last time I did an in-place upgrade (if ever!). I always migrate to new hardware when I'm updating the server OS. You would attach the new domain controller to the domain and copy the Active Directory data as part of the migration.
I feel so thick.... checking the DNS it was not pointing to the server. I have just connected it to the domain succesfully... what a clown
Well I have the 2016 license now, so I guess I will have to reinstall at some point but it is not as urgent now