Apex domains cannot have CNAME records.
Question and response
Question
dig 37.6.219.tellas.gr. NS
Answer
37.6.219.tellas.gr. 5 NS dns2.tellas.gr.
37.6.219.tellas.gr. 5 NS dns1.tellas.gr.
Authority
The response has no authority records.
Additional
dns1.tellas.gr. 5 A 62.169.194.17
dns2.tellas.gr. 5 A 62.169.194.18
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0