Yes, you can use your own router with upc. However, you cannot get rid of the upc box (Connect, Giga, etc.). You have to operate your router behind it. To avoid double NAT (two routers in a row), you can switch the upc boxes to bridge mode (this also deactivates the WLAN). In order for this function to be visible in the menu of the upc box, it must first be switched to IPv4 via the Support Hotline or https://community.upc.ch/.

This is why the Fritzbox in the cable version makes no sense.

I also have no problems with Vodafone DE

Problem-free with vodafone