Apex domains cannot have CNAME records.
Question and response
Question
dig ftth.as8758.net. NS
Answer
ftth.as8758.net. 5 NS ns5.as8758.net.
ftth.as8758.net. 5 NS ns2.as8758.net.
ftth.as8758.net. 5 NS ns3.as8758.net.
ftth.as8758.net. 5 NS ns4.as8758.net.
ftth.as8758.net. 5 NS ns1.as8758.net.
Authority
The response has no authority records.
Additional
ns1.as8758.net. 5 A 212.25.26.193
ns2.as8758.net. 5 A 212.25.2.15
ns3.as8758.net. 5 A 139.162.180.131
ns4.as8758.net. 5 A 172.105.115.196
ns5.as8758.net. 5 A 173.255.213.12
ns1.as8758.net. 5 AAAA 2001:8e0:41:604:0:0:0:193
ns2.as8758.net. 5 AAAA 2001:8e0:80:100:0:0:0:15
ns3.as8758.net. 5 AAAA 2a01:7e01:0:0:f03c:91ff:fe02:cff
ns4.as8758.net. 5 AAAA 2400:8901:0:0:f03c:91ff:fe02:2f09
ns5.as8758.net. 5 AAAA 2600:3c01:0:0:f03c:91ff:fe02:2f69
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0