Cox Communications Uninformed of lease and equipment fee. Casa Grande Arizona

I set up an appointment for new internet service from cox.  When the technician came, I asked him if I could use a used cox modem that I got from goodwill and he said yes, and plugged it in and said it worked fine.  

Several days later, I connected a router and computer to the modem.   The modem started smoking, so I threw it away, and went to Walmart and purchased a new modem.  I called cox and they put in the new serial number for the new modem on my account so I could use the new modem.

When I got my bill, cox charged me a $6.99 charge for leasing that used modem and $122 for the modem itself.  I called to find out why.  A customer service representative took off the $6.99 fee and told me to call back in a few days.

When I called back, the customer service representative said cox won't take off the $122 charge claiming that the modem belonged to them in the first place.  I told the customer service operator that I would like to speak to a manager.  So he put me on hold.  The customer service operator came in on the line and offered to take 1/2 off of the $122 price to settle it before talking to the manager.  I said no, because nobody informed me of any of these charges, I signed nothing agreeing to them.  In other words, I would not have used that modem if I was told I was going to be charged for it.  

When I spoke with the manager, he refused to take off the charge at all.  I said what about 1/2 the $122 that the technician offered.  He said no.  How can cox charge me for a lease that I never agreed to?  Or to pay for a modem that I never agreed to pay for?  I feel scammed by cox.