Apex domains cannot have CNAME records.
Question and response
Question
dig host685-1.slaskdatacenter.com. NS
Answer
host685-1.slaskdatacenter.com. 5 NS ns1.slaskdatacenter.com.
host685-1.slaskdatacenter.com. 5 NS ns2.slaskdatacenter.com.
Authority
The response has no authority records.
Additional
ns1.slaskdatacenter.com. 4 A 178.19.104.11
ns2.slaskdatacenter.com. 4 A 178.19.110.12
ns1.slaskdatacenter.com. 4 AAAA 2a02:2430:3:ffff:0:0:0:10
ns2.slaskdatacenter.com. 4 AAAA 2a02:2430:3:ffff:0:0:0:20
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0