Apex domains cannot have CNAME records.
Question and response
Question
dig mailer.onlineassist.me. NS
Answer
mailer.onlineassist.me. 5 NS dns04.iriscene.info.
mailer.onlineassist.me. 5 NS dns03.iriscene.info.
mailer.onlineassist.me. 5 NS dns01.actualize.es.
mailer.onlineassist.me. 5 NS dns02.actualize.es.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0