Apex domains cannot have CNAME records.
Question and response
Question
dig tertiary.mx. NS
Answer
tertiary.mx. 5 NS ns3.fima-hosting.ch.
tertiary.mx. 5 NS ns1.fima-hosting.ch.
tertiary.mx. 5 NS ns2.fima-hosting.ch.
Authority
The response has no authority records.
Additional
ns1.fima-hosting.ch. 5 A 46.105.191.251
ns2.fima-hosting.ch. 5 A 85.31.153.46
ns3.fima-hosting.ch. 5 A 185.89.147.186
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0