In the Netherlands we now have a law where ISPs must allow your own choice of network equipment. This means they must give you the required information on how to connect your own device with their network. I have a fiber connection, which I connected directly to a Ubiquity router through a suitable SFP module. My ISP supplied the information on the fiber type and which VLAN ID's to setup for internet, TV and telephon…
I have Spectrum cable Internet. I use their modem, but I supply my own router, and they've never given me any trouble. In fact, they recently upgraded my modem (from a Scientific Atlanta 2203C to a Ubee E31U2V1) and they didn't send me a router. The Ubee E31U2V1, like the Scientific Atlanta 2203C before it, only has one Ethernet port, and their official guide to getting the new modem working involved rebooting an ext…
Other cable providers (e.g. Comcast) charge you a monthly fee (~$5-15) to rent their modem. Buying a modem gets cost effective pretty quickly.