Used Modem Activated months ago has now become De-activated and being told I can't activate it now

So my story bought a used Modem from Amazon in September.  At the time it activated just fine showed up on my account.  Was able to stream shows on my phone etc.  Now come December I can't stream on my phone it says can't find Modem.  I login to my account is says Unavailable for my modem. 

So I call Comcast and they say that I can't activate my modem because it's someone else...... but its MINE and I had it activated........ Who can fix this?