I would say this. I also run an FTP server with domain registration through DynDNS.
First, as it sounds you have already done. Is to port forward on your router. Any way, to cover it again open port 21 and forward it to port 21 on 192.168.*.*, whatever LOCAL IP the server is at. If you don't know how to find this, go to Start > Run> Type "cmd" > then type ipconfig > it will be listed as IP address.
After that your routers port should be open so that you can access that port from outside the LAN.
Your problem it seems, is that you don't have a static IP, and that it changes periodically. When you register with DynDNS you use the auto detected IP address I would imagine. After registering try typing in your browser ftp://yourdomain.ftpaccess.cc
It should bring you to a login or FTP screen. That should work. If it works for a while then stops working again, you don't have a static IP. So you would either have to contact your ISP and request a static IP. Or check everyday to see if it changed, and if so, update it in DynDNS. You can check your internet ip at www.whatismyip.com
Hopefully this helps you. If you have anymore questions or it doesn't work, feel free to send me a PM.