Apex domains cannot have CNAME records.
Question and response
Question
dig pool-125-24.dynamic.nt-isp.net. NS
Answer
pool-125-24.dynamic.nt-isp.net. 5 NS ns3.totbb.net.
pool-125-24.dynamic.nt-isp.net. 5 NS ns4.totbb.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0