Apex domains cannot have CNAME records.
Question and response
Question
dig mobile.tre.se. NS
Answer
mobile.tre.se. 5 NS ns2.hi3gaccess.se.
mobile.tre.se. 5 NS ns.hi3gaccess.se.
Authority
The response has no authority records.
Additional
ns.hi3gaccess.se. 4 A 80.251.192.244
ns2.hi3gaccess.se. 4 A 80.251.192.245
ns.hi3gaccess.se. 4 AAAA 2a02:aa0:0:0:0:0:0:53
ns2.hi3gaccess.se. 4 AAAA 2a02:aa0:0:0:0:0:0:54
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0