Apex domains cannot have CNAME records.
Question and response
Question
dig flex.network. NS
Answer
flex.network. 5 NS ns1.flexnetwork.fr.
flex.network. 5 NS ns2.flexnetwork.fr.
Authority
The response has no authority records.
Additional
ns1.flexnetwork.fr. 5 A 185.57.220.177
ns2.flexnetwork.fr. 5 A 185.57.220.178
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0