openwrt router banana pi R3 great for security focus people. its good one but PC and Networks too

Horrible advice your ISP's router is insecure, proprietary spyware. You do NOT want anything untrusted on your local network, let alone your router. Use an OpenWRT router like the OpenWRT One. OpenWRT is free and open source software. No spyware, always up to date, and extremely customizable. Add opnsense if you need more firewall computing power, but that's it.

Maybe Spain has some weird quirks that only specific routers work? Or maybe hebmean modem?

Idk what a hebmean modem is but unless their modems restrict Mac addresses to their specific router then there's no reason you can't use your own router without double NATing. In the worst case scenario, they require you use their own router modem gateway, you're way better off still using your own OpenWRT router and taking the slight double NAT performance hit for the gain in LAN and firewall security. Its still horrible, dangerous advice. Like laughably bad pretty shocking to me.

You could even defeat a modem locked to a specific MAC address router by spoofing your OpenWRT router's MAC address to match so there are always "Cypherpunk" solutions for proper security and privacy.

As I understand it, my ISP DEMANDS that I use a modem running proprietary protocols. I put that sucker in passthrough mode and shoved it into a faraday cage. My main router runs FreshTomato firmware (OpenWRT is an objectively better OS btw, but FreshTomato serves my purposes for now.) (Do you get this notification @Papa Figos ?)