Apex domains cannot have CNAME records.
Question and response
Question
dig wifi-clients.colocenter.nl. NS
Answer
wifi-clients.colocenter.nl. 5 NS ns1.colocenter.nl.
wifi-clients.colocenter.nl. 5 NS ns2.colocenter.nl.
Authority
The response has no authority records.
Additional
ns1.colocenter.nl. 5 A 5.39.184.6
ns2.colocenter.nl. 5 A 5.39.184.7
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0