If you're in Switzerland like the OP, the Swiss government has comprehensive maps of fiber coverage [0] as well as the speed you can attain [1] (there are more breakdowns, check Geocatalog > Population and Economy > Communication.

Even better, you can download the maps and load them into GIS software. I grab listings from real-estate portals then load them into QGIS, where I intersect them with zoning maps, fiber coverage, noise maps and public transit isochrones. I can then filter them at will.

[0]: https://map.geo.admin.ch/?topic=ech&lang=en&bgLayer=ch.swiss...

[1]: https://map.geo.admin.ch/?topic=ech&lang=en&bgLayer=ch.swiss...