The Arris SB6183 is certified compatible by Comcast/Xfinity and ATT (along with the other major cable companies). This modem paid for itself in just a few months and I enjoyed months of additional savings by not renting equipment.
It's simple to use - connect your cable coax to one end port and your WiFi ethernet cable to the other. Power it up and Bob's your uncle! Unlike Comcast/Xfinity's modems which would go out regularly and constantly needed to be reset, this modem never needed to be reset just because, and almost always reset itself after a power outage. No muss, no fuss!
The internal software/firmware automatically updates to the latest greatest by your cable provider.
Why am I selling it you ask? My new provider is fiber optic and I no longer need a cable modem.