Apex domains cannot have CNAME records.
Question and response
Question
dig mob.ru. NS
Answer
mob.ru. 5 NS ns1.ispvds.com.
mob.ru. 5 NS ns2.ispvds.com.
Authority
The response has no authority records.
Additional
ns1.ispvds.com. 5 A 94.250.248.8
ns2.ispvds.com. 5 A 94.250.248.8
ns2.ispvds.com. 5 AAAA 2a01:230:2:75:0:0:0:4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0