Apex domains cannot have CNAME records.
Question and response
Question
dig cloudserver158252.online.pro. NS
Answer
cloudserver158252.online.pro. 5 NS dns3.online.pro.
cloudserver158252.online.pro. 5 NS dns.online.pro.
cloudserver158252.online.pro. 5 NS dns2.online.pro.
Authority
The response has no authority records.
Additional
dns.online.pro. 4 A 217.160.80.248
dns2.online.pro. 4 A 217.160.81.245
dns3.online.pro. 4 A 185.132.34.248
dns.online.pro. 4 AAAA 2001:8d8:fe:53:7072:0:0:1
dns2.online.pro. 4 AAAA 2001:8d8:fe:53:7072:0:0:2
dns3.online.pro. 4 AAAA 2607:f1c0:fe:53:185:132:34:248
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0