Question about LC-Power LC-M34-UWQHD-100-C

I do not get the full resolution with this monitor. In the menu of the monitor it shows me the resolution 1920x1080 @60Hz. I connected the monitor to my MacBook Air 2020 with a Premium High Speed HDMI (4k 60Hz UHD) cable via USB C adapter. According to the manufacturer, the USB-C adapter should be able to transmit 4k video signals at 30 Hz via HDMI. Does anyone have any idea where the problem might lie?