I don’t have fiber access, but at least for cable, my provider (formerly Kabel Deutschland, now Vodafone) allows me to put the modem/router into "modem only" mode, which then allows me to use my own router. Outside of Fritzbox (which is again a whole integrated thing; with questionable features) there aren’t many DOCSIS modems freely available, and the no-name china devices don’t seem much better than my Vodafone Box.

> allows me to put the modem/router into "modem only" mode, which then allows me to use my own router.

Telekom Speedports also have a modem only mode (the ones for non-fiber, dunno about the ones for fiber, but it looked like those are only modems and not a router as well). I don't make use of it since I manage the wifi for my family, but I do know it exists.