AFAIK GPON uses encryption, so you actually get the traffic intended for all your neighbors but can't do anything with it. If you bring your own converter, you wouldn't be able to handle your own traffic either.

Usually yes, but it depends: https://pierrekim.github.io/blog/2016-11-01-gpon-ftth-networ...

Also the authentication might rely on weak secrets. I know my ISP provided FTTH router has a six letter password and a guessable username (derived from my last name), and I can't change either.

Though the research is quite old now. Couldn't find anything recent specifically for DT.

You can bring your own modem. You just have to register it.

But how? There is no information about it, which means, it can't be done without any form of reverse engineering

At least for Germany, you can buy the Digitalisierungsbox Glasfasermodem or any other modem. You just have to register it with the DTAG via their hotline.

This is new to me, i didn't know it was possible now