Not illegal, but the ISPs are seemingly under no obligation to give you those details. In Germany, there’s the “freedom of routers” embedded in the telco law. So they HAVE to give you everything you need to get your custom router online via their wire/fibre.
Bridge mode is just using the ISPs router and bridge that into your router. It’s not the same - you still need the ISP’s access device instead of just yours.
See, in Germany you can buy your own cable modem or fibre endpoint and connect that to the copper wire/fibre line.