Apex domains cannot have CNAME records.
Question and response
Question
dig onrender.com. NS
Answer
onrender.com. 4 NS ns-1324.awsdns-37.org.
onrender.com. 4 NS ns-1829.awsdns-36.co.uk.
onrender.com. 4 NS ns-245.awsdns-30.com.
onrender.com. 4 NS ns-813.awsdns-37.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0