You will likely need or want to keep their modem, but you can build your own private network inside their modem that works better. If you buy you own modem, they will always say "your modem is causing the problem" This is a standard response regardless of the truth. I often recommend you keep their device and built your own network inside it. Just like we do for business.

What's your budget and how big is your house? Single level home? Prewired? Unless you have a small place, a single device will not likely be able to cover everything.

I have deployed a few Meraki Go networks for home users. They work well and are easy to manage. The firewall, switches and wireless access point are separate devices. It is the same hardware we use at businesses with different software. They are not "cheap", but not terrible expensive. You could likely setup you house for the cost of a case of 223 ammo. I have a good vendor if you decide to go this way.