Default DNS as per Registrar

<?php

use WHMCS\Database\Capsule;

if (!defined('WHMCS')) {
    die('You cannot access this file directly.');
}


function pre_register_nameservers($vars) {

    $registrars = [

        'resellerclub' => [
            'ns1' => 'ns1.test.com',
            'ns2' => 'ns2.test.com',
            'ns3' => '',
            'ns4' => '',
        ],

        'connectreseller' => [
            'ns1' => 'ns3.test.com',
            'ns2' => 'ns4.test.com',
        ],

        'epik' => [
            'ns1' => 'ns5.test.com',
            'ns2' => 'ns6.test.com',
            'ns3' => 'ns7.test.com',
        ],

    ];

    /* ------------------------------------------------- */
    /*        ONLY EDIT VARIABLES ABOVE THIS LINE        */
    /* ------------------------------------------------- */

    $domain = $vars['params']['domain'];

    $domainDetails = Capsule::table('tbldomains')->where('domain', $domain)->first();

    foreach ($registrars as $registrar => $nameservers) {

        if ($domainDetails->registrar != $registrar) {
            continue;
        }

        if (empty($nameservers)) {
            continue;
        }

        $nameservers = array_reverse($nameservers);

        $nameservers['domainid'] = $domainDetails->id;

        $command = localAPI('DomainUpdateNameservers', array_reverse($nameservers));

        if ($command['result'] == 'error') {
            logActivity("Automatic nameserver assignment failed for {$domain} with error: {$command['error']}", $domainDetails->userid);
        }

    }

}

add_hook('AfterRegistrarRegistration', 1, 'pre_register_nameservers');