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