Apex domains cannot have CNAME records.
Question and response
Question
dig dyndns.org. NS
Answer
dyndns.org. 5 NS ns4.p201.dns.oraclecloud.net.
dyndns.org. 5 NS ns2.p201.dns.oraclecloud.net.
dyndns.org. 5 NS ns1.p201.dns.oraclecloud.net.
dyndns.org. 5 NS ns3.p201.dns.oraclecloud.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0