Apex domains cannot have CNAME records.
Question and response
Question
dig grupoloading.com. NS
Answer
grupoloading.com. 5 NS ns2.grupoloading.com.
grupoloading.com. 5 NS ns1.grupoloading.com.
grupoloading.com. 5 NS ns3.grupoloading.com.
Authority
The response has no authority records.
Additional
ns1.grupoloading.com. 5 A 91.146.96.9
ns2.grupoloading.com. 5 A 91.146.97.9
ns3.grupoloading.com. 5 A 168.235.68.152
ns1.grupoloading.com. 5 AAAA 2a01:71c0:1:1:2:0:0:1
ns2.grupoloading.com. 5 AAAA 2a01:71c0:2:1:2:0:0:1
ns3.grupoloading.com. 5 AAAA 2604:180:2:15b:0:0:0:329d
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0