Apex domains cannot have CNAME records.
Question and response
Question
dig grt01-1561.grt.ne.allofiber.net. NS
Answer
grt01-1561.grt.ne.allofiber.net. 5 NS den11-adns01.as15108.com.
grt01-1561.grt.ne.allofiber.net. 5 NS blv11-adns01.as15108.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0