I have the same problem -- I have the Davolink DV-2020 DSL modem/wireless/wired/voip router given to me by Tele2. Both iTunes and Firefly on my main pc can be seen by my wirelessly Soundbridge devices if the pc connects wirelessly also, but not if I connect the pc by wire to the router. This is irritating, since the router is right next to the pc (and its wireless card is flaky), but I live with it.
The main Firefly media player page indicates this is likely an issue with multicast IGMP being filtered at the router.
Tele2 has "helpfully" restricted the web interface to the router so there are very few options to pick. Davolink doesn't have a manual (at least not in English) on the device and it wouldn't matter too much.
I found the following information that I hoped would fix the problem:
http://forums.fireflymediaserver.org/vi ... d82b119cd0
If you follow the links you find a list (in Dutch) of various web pages to adjust settings, including IGMP -- http://192.168.1.1/enblservice.html
where 192.168.1.1 is the default internal IP. If I do that, I only get a blank page, and I haven't seen any improvement. I have no clue which of the other links match AP isolation, uPnP, etc.
I'll let you know if I work out anything beyond this -- there must be a fair number of people in The Netherlands who have this problem, since Tele2 has grown a great deal over the last year.
I haven't called Tele2 since I'm a non-Dutch speaking expat here and I didn't think they do much beyond the basic web interface.