If I’m returning a modem that’s been on and off everyday for the past few months, do I need a receipt of that modem or can I just give the account number to the representative?

You don't necessary need a rct they are pretty good about exchanging

If you call customer service for your account, they'll try and do a diagnosis. If they deem your modem to be damaged, they'll direct you to a store and you'll get a reference number. As long as the modem is one from Xfinity, they'll scan it in and replace on the spot for you. I just had to do this process a couple months ago.

No. They scan the modem and it pulls up your account.

I would call the customer service first to make sure. It depends on the modem type if they have it available for exchange.

My experience is call ahead to make an appointment. I returned a modem from my brother and the rep could not find this particular modem. I had a copy of his monthly bill and they exchanged the modem for a router. If you are keeping Comcast no problem. If you are discounting service I strongly suggest getting a receipt