[Soundy]

You need a modem to "convert" the ISP's connection (DSL or cable) to an internet connection (before someone jumps in: no, it doesn't "convert" per se, just trying to simplify). The router performs a number of different functions, including a firewall, NAT translation (so several different devices still look like the same address to the internet at large), WiFi access point, multi-port switch, and so on.

At the very least, you need a modem to connect to your ISP. Most ISPs now are providing devices that combine modem and WiFi router into one box, but most of those, you can also disable the internal router and use only the modem functionality (known as "bridged mode"), then connect your own router.

Whether that will help depends on where the issue is with your setup: the connection itself, the modem portion, the router portion... if you're getting poor WiFi performance, you could definitely benefit from setting the Siemens box to bridged mode and connecting your own router.