I get a similar message, though mine is accompanied by a troubleshooting message of "Your current equipment can’t keep up with your Internet speed. Use our online tool to find modems best suited to your plan."
Guys, c'mon. This is a brand-new Asus CM-32 AC2600. It's more than capable of keeping up with my gigabit plan.
You won't be able to get the gigabit speed tier with that modem no matter what the manufacturer's speed rating is. It's only a DOCSIS 3.0 capable device. Comcast requires a DOCSIS 3.1 capable device for gig speed.