Greetings, Steffen.
I see that you're trying to connect your iPad Pro with your ThinkVision P27u-10 27 inch Wide UHD IPS Monitor. The problem is, it's not showing in fullscreen or 4k resolution, and I'm happy to help.
Charge and connect with the USB-C port on the new iPad Pro
You can connect iPad Pro to an HDMI display or TV with a USB-C to HDMI adapter. Adapters that support HDMI 2.0 can output video from iPad Pro at 4K resolution and 60Hz.
The Apple USB-C Digital AV Multiport Adapter is also compatible with iPad Pro. This adapter can output video from iPad Pro at up to 4K resolution and 30Hz.*
iPad Pro can output Dolby Digital Plus audio streams via HDMI, but not Dolby Atmos. Also note that iPad Pro will play HDR10 or Dolby Vision content via HDMI when HDMI 2.0 adapters that support these formats become available.
* When you use the Apple USB-C Digital AV Multiport Adapter with iPad Pro, its Home screen and apps appear on HDMI displays at 1080p and 60Hz. When you begin playing video content, the resolution switches to match the content, up to 4K at 30Hz. Turn on the Match Content option on iPad Pro in Settings > Displays.
Take care!