owned and still own my cisco linksys DPC3008 MODEM

I purchased my cisco linksys DPC3008 MODEM from Amazon in 2014. I switched from residential to business. From the 1st day the comcast tech told me that Comcast had my modem as a leased modem. He called and tried to fix the issue but said I would need to provide proof that the modem is my modem. I provided proof with Amazon invoice, modem retal box and MAC address to Comcast. The issue was fixed at that time. Now every year at random times I get billed for equipment rental fee. I am having to do the same again. Why does this keep happening?