So in this flat I have a fiber connection, with a PON. I plugged in my own Archer C6, flashed OpenWRT on it, and flubbed with configuration until I got it to work (I wasn't very familiar with OpenWRT, ...

I've always been a fan of putting aftermarket firmware on consumer-grade routers. Whether it's DD-WRT, Tomato, OpenWRT or whatever your favorite flavor of "better than stock" firmware might be, it ...