Wireless signals from what?
The Fon box is simply another router/AP on your network.
You need to set up the Fon WiFi SSID/password with a PC connected to it on the wired PC port.
Leave it as DHCP so it picks up its own IP from your Belkin and dishes out IPs to WiFi clients - by default these will be in a different subnet so there will be no conflict but the machines connected to it will not be able to access your network.
Once that is done connect the Fon Wan port to one of your Belkin LAN ports and it should all just work.
As I said this assumes your newer Fon Simpla is basically the same as my Fonera+
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Here are the official Fon Simpla setup instructions
http://corp.fon.com/en/products/simpl/setup/ They assume you don't want to set up your own SSID/password. One thing I did forget is that the Fon box will update its firmware when first connected - maybe you were not waiting long enough?