To use a private nameserver, At first, you must register/make the Private Nameservers.
First, you need to check your hosting’s DNS IP. How do I find the hosting’s name server or IP? Follow this article for that: –https://support.hostever.com/kb/how-to-find-nameserver-bangla/
Now go to the Private Nameservers option in your domain control panel, now save your custom name in the first box of Register a Name Server and your hosting IP in the next box. If necessary, follow the screenshot below.
Similarly, register the second name server.
Note: When registering the first name server, you must use the first IP of the hosting name server and the second IP next time.
Now set these two custom or private name servers as your domain name servers. Like the screenshot below. You can check if your domain name server has been updated here: https://www.dnslt.com/
Then you need to add two A records from the Zone Editor option of the cPanel or your DNS management of Domain.
1. Enter your first custom / private DNS in the Name box.
2. Type A must be selected
3. In the Record box, enter the IP you used when registering that private name server.
4. There will be TTL or all other defaults.
Follow the screenshot below.
Add the second one in the same way.
Congratulations, work done, your private name server has been successfully added.