In the US you usually need to use your ISP's modem. Even if you buy the modem, it needs to be one that the ISP supports and the ISP will have more control of the device than you do. Even if it were running openwrt or ddwrt, you would not have access to use it.
I have an Arris modem and it works fine now, but for months there was a bug where it would randomly crash. I don't know when the bug was fixed, but firmware updates are controlled by the ISP so I had to just reboot it when it would crash. In other words, even if you have good modem hardware, whether it works correctly is up to your ISP.