If you can access the Apex using the ISP-assigned IP address, but not using the dyndns hostname, then the problem probably lies with the DDNS process.
Sign into the DynDNS web site. Go to My Services->My Hosts, then your hostname entry. Make sure that the record has not expired. Then, note the IP address listed.
Open a new browser window or tab, then go here:
http://www.reeftronics.net/showip.php
Compare the IP adress shown with the one in DynDNS. They should be the same. If not, then the DynDNS update function in your router (or other update method) isn't working.
If they match, then the problem lies elsewhere.