mts.openenglish.com has no NS records.
NS records for this zone are defined at openenglish.com.
NS records for this zone are defined at openenglish.com.
Question and response
Question
dig mts.openenglish.com. NS
Answer
The response has no answer records.
Authority
openenglish.com. 900 SOA ns-507.awsdns-63.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0