Being charged for an inactive device I no longer have access to since July

I lived in an apartment complex that required us to use Xfinity internet after we had been using a different provider. After moving out in June, I was instructed by the Comcast technician that helped with the installation at that apartment to leave the device for the next tenants, as he went ahead and set me up to receive a new gateway router at the new apartment. Upon setting the new device up at the new apartment, I am still being charged for the original router as well as my current router with on the bill statement with a message saying “This equipment is inactive. We’ll be happy to remove this charge once it’s returned to a local Xfinity store” The problem here is that I can’t return the device as I no longer live in the same area and was instructed by the Xfinity representative to leave the device. I have reached out to support, who said that until I returned the device, I would be charged, even after explaining the situation that I was told by Xfinity not to return the gateway. I also contacted the representative who claims to have gotten it “fixed” for a few months now, but I still have been overcharged since July. What can I do to get rid of this charge?