Apex domains cannot have CNAME records.
Question and response
Question
dig virtua.systems. NS
Answer
virtua.systems. 5 NS ns3.sdns.network.
virtua.systems. 5 NS ns4.sdns.network.
virtua.systems. 5 NS ns1.sdns.network.
virtua.systems. 5 NS ns2.sdns.network.
Authority
The response has no authority records.
Additional
ns1.sdns.network. 5 A 185.10.18.18
ns2.sdns.network. 5 A 185.154.152.18
ns3.sdns.network. 5 A 128.199.56.129
ns4.sdns.network. 5 A 149.248.57.16
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0