Apex domains cannot have CNAME records.
Question and response
Question
dig cetrel.lu. NS
Answer
cetrel.lu. 5 NS ns3.as8677-dns.net.
cetrel.lu. 5 NS dns3.p03.nsone.net.
cetrel.lu. 5 NS dns4.p04.nsone.net.
cetrel.lu. 5 NS dns1.p01.nsone.net.
cetrel.lu. 5 NS ns1.as8677-dns.net.
cetrel.lu. 5 NS dns2.p02.nsone.net.
cetrel.lu. 5 NS ns4.as8677-dns.net.
Authority
The response has no authority records.
Additional
dns1.p01.nsone.net. 2 A 198.51.44.1
dns1.p01.nsone.net. 2 AAAA 2620:4d:4000:6259:7:1:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0