Apex domains cannot have CNAME records.
Question and response
Question
dig ironmountain.com.pl. NS
Answer
ironmountain.com.pl. 5 NS ns1.ironmountain.com.
ironmountain.com.pl. 5 NS ns2.ironmountain.com.
Authority
The response has no authority records.
Additional
ns1.ironmountain.com. 5 A 216.229.153.20
ns2.ironmountain.com. 5 A 216.229.145.31
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0