It is a Speedport Smart 3 from "Deutsche Telekom" . But I checked all ports, and I found the following problem: It wasn't in a LAN port yet. So, I solved that.

I updated the firmware information in the post. The information is from the Tether app, so it's the the version name is exactly. That's so, because i can't access the information from the computer...

No, it doesn't look like this. It is the EU version.

But I'm on an older page. I'm lucky that I'm using the Tether app.

I connected a Ethernet cable to my C20 and my modem. The C20 didn't and doesn't know, that a modem is connected. It seems like there is an error. Please Help me tp-link!