Apex domains cannot have CNAME records.
Question and response
Question
dig tecnologiasplexus.com. NS
Answer
tecnologiasplexus.com. 5 NS isp-01.plexus.es.
tecnologiasplexus.com. 5 NS dns4.plexus.es.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0