Apex domains cannot have CNAME records.
Question and response
Question
dig recruiting-portal.com. NS
Answer
recruiting-portal.com. 5 NS ns1101.ui-dns.biz.
recruiting-portal.com. 5 NS ns1056.ui-dns.com.
recruiting-portal.com. 5 NS ns1060.ui-dns.org.
recruiting-portal.com. 5 NS ns1118.ui-dns.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0