Apex domains cannot have CNAME records.
Question and response
Question
dig dynamic.cust.swisscom.net. NS
Answer
dynamic.cust.swisscom.net. 5 NS ns12.scs-ns.org.
dynamic.cust.swisscom.net. 5 NS ns11.scs-ns.ch.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0