I have the exact same configuration as you, same router, same nas and same FW (on both!). It works great. Just check out to config ddns on the nas and do port fowarding (on the router) to the nas (in my case, I fowarded all ftp connections on my specific FTP port, wich is listening the NAS, to my NAS. For example the default one is 21). Feel free to PM me for any further instructions or questions.