If you look at _msdcs.mydomain, do you see any records?

Make sure your DC Resolver settings are pointing to the IB DNS Primary.

Did you add the DCs IP addresses to 'Allow Update' or 'Active Directory' in the Grid or Zone Settings?

(or do you use GSS-TSIG?).

You don't need to add a Host-Record for the DC. The DC will register itself, when Updates are allowed.

Test Updates via 'ipconfig /registerdns' for the A Record of the DC. Review the Logs of the Grid DNS Primary.

Use 'nltest /dsregdns' to force the DC to send the SRV Records to DNS. Also review logs for error or success messages.