If the above does not work, try this.
If you have a digital voltmeter you could look around the garage, or attic to see if you find the outside cable feed and the connection to a splitter or coupler. Take this apart and place a jumper from the center lead to the outside metal coupler, then you can go room to room with the DVM and check for a short circuit. Remove the jumper, reconnect the cable and install your modem.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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