Apex domains cannot have CNAME records.
Question and response
Question
dig 48-29.108.20.62.in-addr.arpa. NS
Answer
48-29.108.20.62.in-addr.arpa. 4 NS dns1.tankbar.com.
48-29.108.20.62.in-addr.arpa. 4 NS dns2.tankbar.com.
Authority
The response has no authority records.
Additional
dns1.tankbar.com. 4 A 195.67.74.178
dns2.tankbar.com. 4 A 51.12.242.14
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0