Getting charged for a Storm Ready modem after disconnection that the store took

So I recently disconnected my Xfinity internet after 25 months. I returned my gateway and storm ready modem thing to my local in person xfinity store. The employee took the storm ready modem along with my gateway. I was shocked when I saw I was still charged for something on a bill, and saw it had something to do with the storm ready wifi. I assumed it may have been because they didn’t mark that I’d returned the equipment but came to learn that no, it’s because the storm ready wifi modem is actually sold to us under a lease (which I wasn’t aware of, I was under the impression that I was renting the equipment like my gateway) and that the charge was a lump sum of the remaining months of the leased equipment. That’s obviously already annoying to begin with and is definitely a “gotcha” marketing strategy to get more money out of people; however, the store employee took my equipment and did not make me aware of this. So now I’m not in possession of this equipment that I apparently own and in fact xfinity itself is now in possession of it. So I’m wondering how to go about getting this charge taken off my account.