Apex domains cannot have CNAME records.
Question and response
Question
dig static.speed-net.com.pl. NS
Answer
static.speed-net.com.pl. 5 NS ns1.speednet.com.pl.
static.speed-net.com.pl. 5 NS ns2.speednet.com.pl.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0