I've done a bit of googling, but it appears this is an eastern european product? so all the manuals I find are in Russian or other languages I don't speak.

All home routers I've come across have a firewall that blocks most incoming connections but allow all outgoing connections. As your N900 will be on your home network it will be initiating the outgoing connection so it should be open.

You shouldn't need to port forward ports 110 and 25 as that's probably for incoming - and as it hasn't made any difference I'd delete those entries.

One thing I'm wondering is if your router offers some kind of guest mode on your WiFi which might be heavily restricted. Have you entered a WPA key/password on the router? Did you get prompted for it when you first connected to the WiFi?