I had the same situation with Comcast about a year ago. I can’t stand something that only works half-assed; so when they told me I needed a new modem I got one. The Motorola I’d used for 10 years or more cost me $0. I forget the cost of the modem it’s been so long ago, but there was a mail-in rebate which made the modem For Free.
This time Comcast said I could rent a new modem from them, or go buy one. I bought one at Best Buy, unhooked the old one, hooked up the new one and had Comcast Tech Support pronounce their blessing on it electronically through the internet, and it works perfectly with the latest Comcast technology.