Your best bet would be to setup private nameservers Freenom unfortunately doesn't have any help articles on how to do it, and I have never used them. But if you go to the domain, and select nameservers, there should be an option for "Private Nameservers" where you can put both ns1.mydomain.com and ns2.mydomain.com with your servers IP. Then set any domain you host to ns1.mydomain, ns2.mydomain.com as their nameservers as well.