Apex domains cannot have CNAME records.
Question and response
Question
dig gdnsec.com. NS
Answer
gdnsec.com. 4 NS radium.gdnsec.com.
gdnsec.com. 4 NS tin.gdnsec.com.
Authority
The response has no authority records.
Additional
tin.gdnsec.com. 4 A 45.116.40.1
radium.gdnsec.com. 4 A 45.116.40.2
tin.gdnsec.com. 4 AAAA 2404:ae00:3266:0:0:0:2d74:2800
radium.gdnsec.com. 4 AAAA 2404:ae00:3266:0:0:0:2d74:2801
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0