GTM - DNS question- subdomain

Thanks for the response - I did test t(doing it the wrong way) on the GTM and it does appear to work as you mentioned, however if you update a record (e.g. change the IP of a A record) it creates a duplicate record in the zone and leaves the old record untouched.

 

 

I am presuming on the GTM's I would simpy define NS records in the main zone (domainname.com) for the subdomain zone, i.e.country.domainname.com and then create a new zone in zonerunner to add the A records to?