On enom's menu there's a radio that disable all features if is not set to "our servers"...
but on WHMCS, instead of seeing it like this, we see the dns names, no radio, and no Disabling the features, wich would be just fine, because there's no way you can handle all name servers, but it's wrong to just show the features as if they'd work when they don't. And it feels wrong to just add a reminder "this won't work unless NS are set as... " or I'm just gonna put a javascript wich disables some buttons if NSs aren't the default ones, but is an ugly solution.
I'm also curious about what register NS does. My guess was it just added a host record for the name server. But I don't think that's what it does, since the Manage DNS reads all host records apparently (even those you set up as mail forwarding), and this NSs do not appear. In the eNom's API Command Catalog still clears nothing about this feature for me, asuming it's using the RegisterNameServer command.