Apex domains cannot have CNAME records.
Question and response
Question
dig cust.as8218.eu. NS
Answer
cust.as8218.eu. 1 NS nsauth2.dns.neotelecoms.com.
cust.as8218.eu. 1 NS ns50.zayo.com.
cust.as8218.eu. 1 NS ns60.zayo.com.
cust.as8218.eu. 1 NS nsauth1.dns.neotelecoms.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0