OK, little bit of a lie when I said I never had them working reliably - I also had my Foscams plugged into an external network as well, and they worked fine, BUT I got told off when I asked for help with my VOIP (which I still can't get working, despite having a "normal" network setup again). Have to keep my setup as textbook as possible or I will never get ANYTHING going. So no more external router.
I remember discussing the Foscams on IRC, and the words "ARP" and "stack" and "crappy" were being used. And whilst that might be a true, standard routers do seem to be able to handle them (I'm sure I'm missing something fundamental as to why that is). Maybe it's the same story with your old satellite receiver.