Apex domains cannot have CNAME records.
Question and response
Question
dig thermoking-bg.com. NS
Answer
thermoking-bg.com. 5 NS ns1.s8.r1servers.com.
thermoking-bg.com. 5 NS ns2.s8.r1servers.com.
Authority
The response has no authority records.
Additional
ns1.s8.r1servers.com. 5 A 164.138.218.12
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0