The signal on the coax cable is converted to Ethernet with a cable modem. So TWC could set this up for you. You can't split the coax since cable modems have to be activated by the ISP.
Call them and see how much.
Did you try a better WiFi device? Like this one?
http://www.amazon.com/Etekcity-Wireless-Network-Adapter-Antenna/dp/B006JWMOOI/
I live in an upstairs apt. My landlord lives below. TWC ISP has a coax cable that goes from outside and into my bedroom but then the coax goes down through the floor and into the modem in my landlords apartment. I can't run an ethernet cable down to the modem so could I do anything with a splitter and have another coax route to a router or something? or is the only way through the single TWC provided modem downstairs? Wireless on my desktop via wireless adapter just isn't cutting it for online gaming.

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