A change of Name Servers will change the location from where your domains Zone Records are requested from.
This means that if you are assigning for example ns123.example.com as your name servers, all records have to be set up on those name servers for your domain to work as intended.
To delegate a domain to different Name Servers:
1. In your dashboard go to Domains - My Domains;
2. CLICK on the domain name you wish to modify;
3. SELECT the 'DNS Settings' tab;
4. SELECT 'Delegate to Your Name Servers';
5. Enter the new Name Servers (be sure to 'Add' after entering the first two name servers and for each additional name server you enter if you have more than two);
6. SELECT the 'Delegate to Your Name Servers' option. You will see an on-screen confirmation that the DNS has been updated successfully.
Please allow around 24-48 hours for propagation of any DNS changes, delegations and/or web re-directions across local name server caches for them to be visible globally.