Comment by NekkoDroid

> 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.