Ok. I do see somethings that are incorrect.. I had to download the manual for your router and check a few things, but I think I've got it all covered.
So to start you need to disable your routers built in FTP server. It should be under the USB extension menu on the FTP share table. Just disable it completely because your NAS will handle the web browser and FTP explorer.
After that you will need to change the port. If your NAS is setup exactly like the link I posted for you then you only need to forward port 21 to your NAS IP. (if it won't let you just put in port 21 then forward 20:21 and don't forget that : between them : ). The destination (local) IP is the NAS IP.
Alright now from here I think you should change something with your DDNS. If your followed the seagate guide then you have your DDNS updater on the NAS. I would disable that and use the DDNS updater on your router. It should be setup just like the NAS. If you need a step by step for it just let me know.
Ok this bit is a little complicated. To access this from a web browser you have two options. The first is to use ftp://(your_ip/address). The ftp:// instead of http:// should automatically try to go through port 21. That doesn't always happen though. The next way to connect would be to put http://(your_ip/address):21
The :21 will force the traffic to go through port 21.
Using http://(your_ip/address) is actually going to port 80 and that is the normal webserver port. If you type that in and try to access it from outside your network one of three things is going to happen. Either the request will time out, say 401 forbidden, or end up showing your login screen to your router. If it shows the login screen then you have a problem. You will need change the setting in your router to ignore requests on 80 and 8080.
That should cover it for you. There are a couple of other options you could setup to change how you access the ftp, but lets make sure it is working now.