Use Asus WL500 Premium V2 like a WIFI dongle or card?

I have an Asus WL500 Premium 2, running Tomato firmware, that I previously used to wirelessly access recordings on my Topfield TF5800 PVR.

I'm sure i remember reading somewhere that the Asus can be used like a Wifi dongle or card to connect wirelessly to the internet, but can't work out how to do it and I am very out of my depth with this tecnology. I want to use it to get online on a desktop PC that is upstairs (connected via Ethernet to the Asus) and the WiFi router and phone socket are downstairs.

Thanks in advance, and i hope somebody can help.