What OS are your desktops using?
One of the limitations of Windows 10 Home vs Windows 10 Professional is the ability to join an AD domain.
With Windows 10 Home you may be able to use some resources on the domain (like connecting to a share with the domain user / password) but full domain membership won't be available.
When you set up Zentyal as a domain controller it is default behaviour to have dns running on it and becoming the gateway for your machines. You can use a different gateway and configure dns forwarders on Zentyal if you prefer to use another device.
As an example, you may want to configure your Zentyal local dns to hold the names and IPs of your machines, but configure one or more dns forwarders (say, Google's dns 8.8.8.
so that any dns query that is not for your local machines can be resolved by Zentyal via the forwarders. Do this and your computers will be able to resolve both local and internet addresses.
If you are adding a Windows 10 Professional pc to the domain, you will need the Zentyal Administrator credentials to perform the domain join; after that you should be able to login to the pc with any of the users you have configured in the Zentyal Users and Computers module.