Please realize that the DC is NOT on the IP that the Nethserver uses. It is actually on the IP of Nethserver +1
This royally screwed me over for half a day.
I have no issues at all getting Free- or TrueNAS to utilize tha SAMBA4 AD running on Nethserver. I do not even require encryption atm.
If you want to make sure that you use the right IP, set your DNS server on True/FreeNAS to the Nethserver, and use **ntds-**nethserver-fqdn as the hostname.
So when your Nethserver is named foo on domain bar.com, you would enter ntds-foo.bar.com and use foo.bar.com as DNS server.