Apex domains cannot have CNAME records.
Question and response
Question
dig de.umantis.com. NS
Answer
de.umantis.com. 4 NS ns6.umantis.com.
de.umantis.com. 4 NS ns1.umantis.com.
de.umantis.com. 4 NS ns2.umantis.com.
de.umantis.com. 4 NS ns5.umantis.com.
Authority
The response has no authority records.
Additional
ns1.umantis.com. 4 A 185.136.96.77
ns2.umantis.com. 4 A 185.136.97.77
ns5.umantis.com. 4 A 185.136.98.77
ns6.umantis.com. 4 A 185.136.99.77
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0