Apex domains cannot have CNAME records.
Question and response
Question
dig ip-51-91-131.eu. NS
Answer
ip-51-91-131.eu. 5 NS dns20.ovh.net.
ip-51-91-131.eu. 5 NS ns20.ovh.net.
Authority
The response has no authority records.
Additional
ns20.ovh.net. 4 A 5.135.43.97
dns20.ovh.net. 4 A 5.135.249.213
ns20.ovh.net. 4 AAAA 2001:41d0:b00:f300:0:0:0:2
dns20.ovh.net. 4 AAAA 2001:41d0:d00:f900:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0