Apex domains cannot have CNAME records.
Question and response
Question
dig assentportal.com. NS
Answer
assentportal.com. 5 NS ns-1907.awsdns-46.co.uk.
assentportal.com. 5 NS ns-508.awsdns-63.com.
assentportal.com. 5 NS ns-774.awsdns-32.net.
assentportal.com. 5 NS ns-1178.awsdns-19.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0