Unwittingly bought returned modem

I bought what I believed to be a brand new router/modem from Walmart to set up my internet - I'm a new customer. However, I moved right afterward and have only recently got around to setting it up. I was struggling with that a lot, and eventually reached out to an agent and I was told the MAC address was already linked to another customer. I'm really confused because this device was purchased from a retailer not second hand and it was packaged and everything. I'm outside of the return window on this device now, and I'm really disappointed. Is there anything that can be done? I have a receipt from Walmart to prove I bought this item, and I really just want to be able to use it and not be out the money from someone else returning their item and not removing it from their account.