I have a Vigor 2920n with two synology NAS (one DS-212+ and one DS-209). UPnP is functioning well for a few minutes after the router has been switched on, but afterwards, the media server, itunes server and the router icon will altogether disappear in the network and can never be detected again. Files browsing can still be assess in "my network" though. I had tried to disable and enable the NAS's itunes server function, DLNA function and router's UPnP broadcasting function, but still no luck on getting it work again. I haven't switched on the Bind IP to MAC and DDoS defence, so I think these are not the causes of my problem. Can anyone help to provide me a solution? Thanks very much.