I had this very frustrating problem for a long time and could find no good reason for it.
I added another Diskstation and noticed the DNS server the second NAS was using was different to my first one. I investigated within the router and found that the ISP had changed my DNS server address. As soon as I updated my DNS server in the network settings it immediately detected the external address again.
I feel a bit stupid, it should not have taken me this long to work that out! Hope it helps someone