Apex domains cannot have CNAME records.
Question and response
Question
dig polyfer.at. NS
Answer
polyfer.at. 5 NS ns3.kapper.net.
polyfer.at. 5 NS ns2.kapper.net.
polyfer.at. 5 NS ns7.kapper.net.
polyfer.at. 5 NS ns5.kapper.net.
Authority
The response has no authority records.
Additional
ns2.kapper.net. 5 A 94.16.111.51
ns3.kapper.net. 5 A 103.241.67.58
ns5.kapper.net. 5 A 94.136.22.5
ns7.kapper.net. 5 A 144.217.92.144
ns2.kapper.net. 5 AAAA 2a03:4000:51:f2e:0:0:0:51
ns5.kapper.net. 5 AAAA 2a02:ab8:4:0:0:0:0:5
ns7.kapper.net. 5 AAAA 2607:5300:201:3100:0:0:0:4caf
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0