Well...you know how to set up the router then, because it was working before on a PC.
Can you check the router logs to see what shows up, and at the same time, I would also try looking at /var/logs/proftpd.log on the ReadyNAS. In order to do this, you'll have to enable SSH on the ReadyNAS box with the add-on if you haven't done so already, and then login as root.
EDIT: You should also see your LAN logins, and if there are any errors on the ReadyNAS; they'd likely be in /var/log/messages. You might also want to reboot the router, just in case it looks like it saved, but didn't.