I have a new DIR-878 that's connected to my ISP router to provide an internal subnet. I configured some port forwarding to allow clients from the outer LAN (ie., those connected to the ISP router) to connect to those behind the 878. However, while it and its clients can access the internet without problems, and it has a valid IP in the outer LAN, it and its clients cannot be reached from the outer LAN using that address
A quick port scan showed it as having everything filtered, but the web interface (which can only be reached from inside its own subnet) shows IPv[4,6] filtering as "off".