The HP website says
Open the following ports for scanning over a network :
UDP ports: 161, 427 and 137
TCP ports: 9220, 9500 and 9290
I'm not sure which ip address to forward these ports to (probably my PC's ip address since scanning goes from the HP device to my computer).
I'm gonna have a go at these ports when I get home tonight. I may have to forward them on both the Netgear and the Fonera since the info has to pass through both routers before reaching my PC.