As already mentioned, how to configure WAN interface, depends on your ISP. However, as you stated above, your WAN is already working. So there is nothing to change for DynDNS.
Get an account from a dynamic DNS provider. Then you can choce a hostname in given domains like yourhost.dyndns.com.
Configure the Dynamic DNS service in pfSense (Services > Dynamic DNS > Dynamic DNS Clients). If it is set up properly it will update the dynamic DNS at provider every time your WAN IP changes.
So you can configure you openVPN clients to connect to yourhost.dyndns.com. The hostname is ever the same, the IP behind may change.