There ais a dns A-Record for ns2 missing. Please add a ns2 A-Record in the zone mydomain.com.
Then check if you are you able to resolve the domain when you run this command on the server:
dig @localhost mydomain.com
If yes, then the problem might be either a closed firewall (please post the output of "iptables -L") or BIND is configured in /etc/bind/named.conf.options to not answer external queries.