Relocating a fibre-optic router-modem within a home
The basic problem with installing fiber is it's glass, and glass doesn't bend well. That thin white fiber cable has a minimum bend radius of somewhere around 10 cm, so if you pull it tight around a corner the glass inside it breaks. And splices are not easy.

The typical solution is install the fiber modem in the best place for the fiber, and then run a single ethernet line to your router. If it's an all-in-one box you can probably get it changed.

30m is hardly unreasonable for ethernet, but I would buy good cable and avoid running it parallel to electrical lines. By "good" I don't mean "says cat-6 on the bag", I mean good quality cable from a known manufacturer. Go to a supplier not in a shopping mall or main-street-front. You want the industrial-park places.