Private nameservers are what make a reseller business look like a hosting company rather than a middleman. Your customers point their domains at ns1.yourdomain.com and ns2.yourdomain.com, and our name never appears.
Three steps, in this order:
1. Register the hostnames at your domain registrar. Look for glue records, child nameservers, or register a nameserver - registrars all name it differently. Create ns1.yourdomain.com and ns2.yourdomain.com, each pointing at the IP addresses in your welcome email. This step is at the registrar, not in WHM, and it is the one people miss.
2. Set them in WHM under Basic WebHost Manager Setup, so every new account you create uses them automatically.
3. Wait, then test. Glue records can take up to 24 hours to propagate. Test with a DNS lookup tool before you point a real customer domain at them.
Do not move existing customers until step 3 passes. Pointing a live domain at nameservers that do not resolve yet takes the site offline, and the fix has to propagate too.
If your IP addresses ever change, the glue records at the registrar must be updated as well as WHM. That one catches almost everybody.
