Apex domains cannot have CNAME records.
Question and response
Question
dig fueber.org. NS
Answer
fueber.org. 5 NS nsz.fueber.org.
fueber.org. 5 NS nsy.fueber.org.
fueber.org. 5 NS nsx.fueber.org.
Authority
The response has no authority records.
Additional
nsx.fueber.org. 5 A 167.233.1.169
nsy.fueber.org. 5 A 83.243.122.239
nsz.fueber.org. 5 A 80.75.240.139
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0