Reach NAS Outside Network

Yep, I agree avoiding anything under 1024 for the port is better since they are the most used ports and this means the most hacked/scanned, but I wanted to keep things simple.
To avoid 20-21 TCP, you will have to tell outside NAT (internet) to use two ports of your choice (avoid anything between 0-1024) and inside NAT (your network) to use 20-21 TCP (or change it in the NAS conf). Then tell your client the outside port you chose when connecting.

The only other dynDNS service I know is no-ip, don't know if they are free or not, but they used to be at least.

My provider's router won't do the port translation, he only has direct forwarding (one port from one IP to the other, no port translation), I really don't know why since it's Netgear hardware but it seems my provider did a custom firmware which is obviously crappy. Happily I received a Cisco router that will do this perfectly and much much more but I don't think that's the right place to speak highly technical stuff ;)