Well since having a backup modem is pretty critical these days, here's a list of modems to think about. The first thing to ask is how much bandwidth you are buying. For 99% of all users, they are getting way less than 300Mbps, so the lowly basic Docsis 3.0 modems should work. But if you are lucky enough (or rich enough) to have over 500Mbps or even 1Gbps, then you need a Docsis 3.1 modem. The basic question to ask is how many downstream and upstream channels does the modem support.
And these days, the Arris modems are pretty much the standard, so here are some recommendations:

1. Arris SB8200. This is a Docsis 3.1 modem that supports up to 2Gbps up and downstream (as long as your wallet can support paying for that much bandwidth). It is a 32 x 8 (that is 32 channels down and 8 channels up). At a pricey $190, it's amazing and I definitely want to squat in your house 🙂 2. Arris SB6183. This is a 16x4 modem so supports 686Mbps. It's cheap at$67
3. Arris SB6141. This is the base model with 8x.4 for 383Mbps. The price is nearly the same at \$40, so you might as well get the 6183.