Apex domains cannot have CNAME records.
Question and response
Question
dig serverhostgroup.com. NS
Answer
serverhostgroup.com. 5 NS ns2.serverhostgroup.com.
serverhostgroup.com. 5 NS ns3.serverhostgroup.com.
serverhostgroup.com. 5 NS ns1.serverhostgroup.com.
Authority
The response has no authority records.
Additional
ns1.serverhostgroup.com. 5 A 104.236.38.47
ns2.serverhostgroup.com. 5 A 46.101.214.99
ns3.serverhostgroup.com. 5 A 141.164.45.163
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0