UPDATE: 1: Thanks to all of you for helping me out! 2. I just had a chat (on Sat-Eve) w my ISP, they could open a “bridge-port” on their last “Lan-socket” on R1.
That’s the way to go, if not, i’ll go DMZ, correct?

it’s fiber.

No I haven’t. First i got to educate myself what these different modes are…

the ISP locked their router. I have to go the the ISP’s site, login and change settings there… can’t even change DNS on ISP-router.

that’s exactly why.