Depending on where you live you might just let your ISP give you a modem and you can choose the router yourself. ISPs use the TR-069 protocol which allows them to manage and administrate the router they gave you. This is probably what they did when they made the changes you mentioned. It even allows them to analyze your whole home network. Thanks to this they can spy on you if they want to. So my advice is to just get a modem from them and choose your own router because then they can't spy on you. My advice is to use OPNsense, it's open source.