Apex domains cannot have CNAME records.
Question and response
Question
dig customer.gigaclear.net. NS
Answer
customer.gigaclear.net. 5 NS ns0.gigaclear.net.
customer.gigaclear.net. 5 NS ns2.gigaclear.com.
customer.gigaclear.net. 5 NS ns1.gigaclear.net.
customer.gigaclear.net. 5 NS ns3.gigaclear.com.
Authority
The response has no authority records.
Additional
ns1.gigaclear.net. 5 A 46.227.144.39
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0