First, this U4919DW sold in Japan on June 7, 2020 is not registered. Go here, enter the private U4919DW Service Tag. It states ="Register to claim your Support services." Second, some version of DDM or DDPM must be on all systems. To do this you would need a version of DDM for Ubuntu, which was never offered. So once switched from #1 Mac Mini or #3 Windows to #2 Ubuntu, you cannot use the shortcut key.  
I have three different operating system computers connected to my U4919DW:#1 Mac Mini#2 Ubuntu#3 Windows I frequently switch between #1 Mac Mini and #2 Ubuntu. So I downloaded DDPM for macOS on my Mac Mini and tried to set up the "Input switch shortcut key". When on the Mac Mini, the shortcut key works and it switches to my Ubuntu machine. But when I press the keyboard shortcut on Ubuntu, nothing happens? I installed DDPM on macOS Mac Mini and now every time I switch the input to another computer via the monitor's menu, I get a notification on macOS saying, "Dell U4919DW is not responding to messages. Check the OSD settings, cables, and the GPU display driver for DDC/CI support". Prior to installing DDPM, I updated the firmware on the U4919DW.Firmware version: M2B108DDPM version: 1.0.2.0019
DDPM, input switch key not working