I strongly assume that you do not have a modem but a router from UPC or QuickLine.
At least it's been many years since I last saw UPC deliver a pure modem. And that makes a difference with regard to your question.
You now have two options:
1. switch your cable router to "modem mode" or "bridge mode" (then it behaves like a modem), but with the latest UPC routers this only works under certain conditions. (See, among other things,
https://support.upc.ch/app... )
2. you switch your FritzBox to IP client mode (see link from Anonymous), but in doing so you lose some of the functionality of the FritzBox.