Apex domains cannot have CNAME records.
Question and response
Question
dig static.t-mobile.at. NS
Answer
static.t-mobile.at. 5 NS ns2.t-mobile.at.
static.t-mobile.at. 5 NS ns1.t-mobile.at.
Authority
The response has no authority records.
Additional
ns1.t-mobile.at. 5 A 212.166.122.200
ns2.t-mobile.at. 5 A 212.166.123.200
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0