Apex domains cannot have CNAME records.
Question and response
Question
dig sms.at. NS
Answer
sms.at. 5 NS ns2.easyname.eu.
sms.at. 5 NS ns1.easyname.eu.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0