The router can be seen as an extension. You can connect another modem to it and establish the Internet connection via cable LAN. I also tried it with WLAN, but that doesn't work.

The modem of the provider must be attached first, then the RX500 😉 Yes, the RX500 can be fully configured.
Yes, the RX500 can be fully configured and does not weaken one bit 😀

I'm with Sunrise and it didn't work to connect it directly to the DSL socket. But it works after the Fritzbox.