Per Dell-Cares: The USB C port on the AIO is to connect to external storage devices and printers. So in this case, you would be unable to connect a monitor through the USB C port. You can use the HDMI-out port to connect to another monitor.
Can Inspiron 5400 All-in-one be connected to a usb-c monitor?  Specifically, I tried connecting to U2419HC through a USB-C cable.  The monitor does not get a video data signal on the USB-C input source.
Inspiron 5400 AIO USB-C to U2419HC