Isn't possible with reverse proxy so he don't need open up a extra port?Well configure a cname record that will point to your ddns name on the nas. Open up a port on your router to forward traffic for the ftp port towards your nas. Use an ftp client to connect using the alias name and the port number that you have configured.
Where your domain is hosted you can (should be able to) create a cname record and point it to your static ip address. Problem solved.@Rusty was able to get a static ip from my isp (apparently they changed the rules). In regards to the CNAME. I use Migadu.com for email and I have to point my nameservers to those from Migadu. Does means that I can not change the DNS records anymore. Should I temp switch the godaddy DNS recoards add cname etc?