Make TC7200 U modem/router usable again

There is a Technicolor TC7200.U left from my last flat and inteet provider. This one is still configured like the inteet provider prepared it in the first place, which means that I can't use it in my new flat.

How can I make the modem/router usable (for anything) again? Has anyone of you an idea or for instance knows a tutorial to replace the firmware on it?

I wasn't able to find a lot of information about the TC7200.U in the last hours and nothing usable to make the modem/router reusable. Does one of you has an idea?

Thanks for thinking about the question ;).