Sorry for the dumb question and hopefully this is relevant enough to the sub. I have my own firewall and right now it connects to my ISPs provided home router over rj45, their router gets a fiber hookup to their network and it’s the only ISP device in my home. If I have a firewall with a fiber port, can I take the fiber to the modem and hook that straight to my firewall, or is there a reason I need their device?
As others mentioned, it depends. Germany has freie Routerwahl by law and ISPs have to allow you using your own router. They won’t offer support configuring it if course but they usually provide you with some credentials to properly authenticate.