I recently changed to another isp zo I also got another modem (Cisco EPC3925)
The router in the modem I had before (fritzbox) was able to redirect an external port (lets say 6000) to internal port 80 and internal ip I have for the DS410 (firmware 3.1) that I'm running.
The worked just fine. I had to do this because I'm running another device on port 80 and I can't change that.
The new router can only route an external port directly to the internal ip address so I added the port 6000 in webservices/http-service. When I try to access the photo station from the outside
using http://my-ip:6000/photostation I get a Synology page "Sorry, the page you are looking for can not be displayed" or something to that effect (don't know the exact translation since I'm running the Dutch language) So it seems that I am getting through to the DS from the outside, but it just won't start the photostation.