can't add users from Domain

I have 2 servers.  Server A is running AD, DHCP, DNS etc...and shows as healthy.  No errors in Server Manager and no errors in EventLog.  Server B is running 2016.   Yes, it is joined to the Server A domain (called lab.net)  

From Server B I go into USERS.  I try to add the Domain Admins from Lab.net but it does not see lab.net.  It only sees itself.

what can cause that?   

mqh7