Yes, you need to point them to your external IP address. I don't believe that you can do what you need on the dynDNS free service - they do have a paid service for that feature. I use a DNS service from
www.easydns.com - they're moderately priced, excellent customer support and a very easy control panel to set-up your DNS plus they have the facility to point to a dynamic IP. I have no affiliation to easydns other than as a happy user.
You'll still need the internal DNS server because you're behind a NAT router.