DNS/NameServers are set from within WHM (Web Host Manager), not CPanel specifically so you would need to log-in to WHM to actually set the DNS for your domain.
Once you login to WHM, you should see an option on the left side-menu: Basic cPanel/WHM Setup which will allow you to set the DNS. You may also need to edit the DNS Record for your domain if WHM does not do that for you.
You would also need to create/register the NameServers with your domain registrar using the IP Address.
To Setup Nameservers In WHM go into WHM and select Edit Setup from the Server Setup menu on the left. Enter ns1.newdomain.com in the Primary Nameserver field. Hit 'Assign IP Address' (write the IP down or remember it for the next step). Then hit 'Add an A Entry for this nameserver'.
Repeat this process for the Secondary Nameserver field. Hope this might help you.
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