XPS 9575, S2725QC, USB-C maximum 120Hz screen refresh rate only at a resolution of 2560x1440

Why is the 120 Hz refresh rate only available at 2560x1440 resolution? While when I set it to 3840x2160, it only goes up to 60 Hz? Should I switch the external monitor from the integrated graphics card to my discrete one? However, I can't find an option to switch from the integrated graphics to the discrete graphics on my laptop when connecting an external monitor.