Apex domains cannot have CNAME records.
Question and response
Question
dig luco.restena.lu. NS
Answer
luco.restena.lu. 5 NS ns3.restena.lu.
luco.restena.lu. 5 NS ns1.restena.lu.
luco.restena.lu. 5 NS ns2.restena.lu.
Authority
The response has no authority records.
Additional
ns1.restena.lu. 5 A 158.64.1.23
ns2.restena.lu. 5 A 91.92.119.210
ns3.restena.lu. 5 A 158.64.12.45
ns1.restena.lu. 5 AAAA 2001:a18:1:0:0:0:0:23
ns2.restena.lu. 5 AAAA 2a04:c46:e00:e7a1:477:a6ff:fe00:afb
ns3.restena.lu. 5 AAAA 2001:a18:6:2:0:0:0:45
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0