So I wanted to change DNS settings on my router but when I tried to access the router’s configuration page (via the gateway IP), the connection is refused.

The router is ISP provided, and internet doesn’t work if the fiber optical cable is connected through another device, so I assume that they have a custom firmware installed. Resetting the router does grant me access to the configuration page but the internet doesn’t work and I have to call the technical support of my ISP.

So my question is, is it normal for ISPs to block access to router’s webpage? Also, is there any other way to access the configuration page if I want to change some stuff there?