Apex domains cannot have CNAME records.
Question and response
Question
dig smartguard.io. NS
Answer
smartguard.io. 5 NS ns2.smartguard.io.
smartguard.io. 5 NS ns5.smartguard.io.
smartguard.io. 5 NS ns3.smartguard.io.
smartguard.io. 5 NS ns1.smartguard.io.
smartguard.io. 5 NS ns4.smartguard.io.
Authority
The response has no authority records.
Additional
ns1.smartguard.io. 5 A 135.125.245.17
ns2.smartguard.io. 5 A 149.28.58.237
ns3.smartguard.io. 5 A 45.63.91.200
ns4.smartguard.io. 5 A 158.247.201.122
ns5.smartguard.io. 5 A 139.99.222.96
ns1.smartguard.io. 5 AAAA 2001:41d0:701:1000:0:0:0:23e6
ns2.smartguard.io. 5 AAAA 2001:19f0:5:3d10:5400:3ff:fe8e:ce9e
ns3.smartguard.io. 5 AAAA 2001:19f0:ac01:1f19:5400:3ff:fe93:86b
ns4.smartguard.io. 5 AAAA 2401:c080:1c02:369:5400:3ff:fe93:85e
ns5.smartguard.io. 5 AAAA 2402:1f00:8102:100:0:0:0:50e
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0