Apex domains cannot have CNAME records.
Question and response
Question
dig assistant.ua. NS
Answer
assistant.ua. 5 NS nsa1.srv53.org.
assistant.ua. 5 NS nsd2.srv53.com.
assistant.ua. 5 NS nsb2.srv53.org.
assistant.ua. 5 NS nsb2.srv53.net.
assistant.ua. 5 NS nsa4.srv53.net.
assistant.ua. 5 NS nsc4.srv53.com.
assistant.ua. 5 NS nsb4.srv53.com.
assistant.ua. 5 NS nsa3.srv53.com.
assistant.ua. 5 NS nsc4.srv53.org.
assistant.ua. 5 NS nsc4.srv53.net.
assistant.ua. 5 NS nsd3.srv53.net.
assistant.ua. 5 NS nsd4.srv53.org.
Authority
The response has no authority records.
Additional
nsa1.srv53.org. 5 A 178.32.247.3
nsa1.srv53.org. 5 AAAA 2001:41d0:203:dee4:178:32:247:3
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0