Hi,
I am a bit confused by your use of internal and external ip. I am also unsure of the router you are using. I would guess that what you call external is the routers ip address and the internal is the ip address assigned by the router to your NAS. Your ddns account should be updating the router ip assuming this is Dynamic, odd that it changes every day though. You should be able to set your NAS to have a static / reserved ip address from your routher so that should not change. If you find your router cannot reserve ip addresses, which is rare, you can try just changing the NAS settings to static from dynamic. But it is far better to do this through the router. The other major part is get the port forwarding working on the router. All of this is pretty clear on the previous youtube videos, so double check every tedious detail and hopefully you will be successful. Good luck.