Apex domains cannot have CNAME records.
Question and response
Question
dig smart-homes-gebelkofen.com. NS
Answer
smart-homes-gebelkofen.com. 5 NS ns1.smart-homes-gebelkofen.com.
smart-homes-gebelkofen.com. 5 NS ns2.smart-homes-gebelkofen.com.
Authority
The response has no authority records.
Additional
ns1.smart-homes-gebelkofen.com. 5 A 216.245.192.100
ns2.smart-homes-gebelkofen.com. 5 A 50.2.42.6
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0