Apex domains cannot have CNAME records.
Question and response
Question
dig tns2.eu. NS
Answer
tns2.eu. 5 NS ns01.cpserver.net.
tns2.eu. 5 NS ns02.cpserver.net.
tns2.eu. 5 NS ns03.cpserver.net.
Authority
The response has no authority records.
Additional
ns01.cpserver.net. 5 A 185.51.188.4
ns02.cpserver.net. 5 A 79.172.252.4
ns03.cpserver.net. 5 A 195.201.215.228
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0