Apex domains cannot have CNAME records.
Question and response
Question
dig simnet.is. NS
Answer
simnet.is. 5 NS ns1.simnet.is.
simnet.is. 5 NS ns2.simnet.is.
simnet.is. 5 NS ns3.simnet.is.
Authority
The response has no authority records.
Additional
ns1.simnet.is. 5 A 157.157.133.114
ns2.simnet.is. 5 A 157.157.133.115
ns3.simnet.is. 5 A 18.202.64.212
ns3.simnet.is. 5 AAAA 2a05:d018:59d:3201:7395:f3be:4cb:af1a
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0