PDC: Not able to connect clients

I have installed a PDC from ISO.

I have gone through the setup process.

I have followed:
https://www.turnkeylinux.org/docs/domain-controller

But when I try to add a client the client says:
Domain not found (Or similar, not an english windows).

I suspect that it is actually found, because if I enter rubbish ind the domain name field, it says "domain not found" at once, but if I enter the correct domain name, it ask for at username and password. After some time, it tells me the same thing.

I have tried with a created user account, created with webmin.
I have tried with "administrator"
I have tried with "root"

All the same result.

Windows 10 pro client.

Any suggestions?