Way to generic of a question. It greatly depends on the type of connection. Somethings like fiber connection are very proprietary.
A lot of the restirctions are to prevent people form hooking up something and not paying for service.
It really shouldn't matter. Most times you can set it to run as only a modem, if you can't the ISP likely will set it that way. You can also just put a second router in front. If wifi is the issue just turn the wifi off on the ISP router.