Apex domains cannot have CNAME records.
Question and response
Question
dig mxmum.freshdesk.com. NS
Answer
mxmum.freshdesk.com. 5 NS ns-810.awsdns-37.net.
mxmum.freshdesk.com. 5 NS ns-186.awsdns-23.com.
mxmum.freshdesk.com. 5 NS ns-1889.awsdns-44.co.uk.
mxmum.freshdesk.com. 5 NS ns-1446.awsdns-52.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0