I have never had that happen, so it’s not normal in that sense. In the sense that it’s the property of the ISP and you are renting it - yeah seems completely normal. They don’t want people messing with config, or they want you to use another method like calling support.

ISPs in the US can harvest and sell your web browsing data. They don’t want you to do anything to endanger that.

There are routers that don't have a local management web page, but instead are fully and only managed through the ISPs CPE (Customer Premises Equipment) management system. It can here be that the ISP have a "my page" on their web or an app for changing the settings on the router.

There are some good reasons for doing this. Looking back the last 10 years+, quite a lot of the security issues that have been on routers have been related to local interfaces (webpage/APIs). So there is a security element in this. You can also deploy changes to the management interface here quite easily, or even add/remove features, without having to change anything in the firmware for the CPE.

If it’s the ISPs router, yes. Otherwise they would have no way to block the routers local site.

Companies like Spectrum not only block local IP login but force you to use their terrible mobile app to even gain access to the router. Even then you’re only allowed to change the Wi-Fi name and password and literally nothing else.

Wouldn't shock me at all.

Always use your own router. Tell the isp to give you a "modem only" or ont, and then use your own device. Give them no access into your network as a best practice.

Are you trying to access it through a wired connection? I can access the configuration page for my isp-provided router but only through a wired connection.