This is correct and working as designed. The system will only work with the RTX card over HDMI and the integrated panel.  The image you shared of the GeForce settings for PhysX also confirms this.  A similar thread for the Alienware R7 discussing the same is located here. Since the system only has Thunderbolt ports and not a USB-C port by itself there is no way around this.   
I have G15 (5520) with RTX 3060. I've read here, and other forums, and even in a youtube video review that USB-C/DisplayPort output is meant to be driven by RTX 3060 discrete GPU. Meanwhile the HDMI output will be driven by the integrated Intel Iris. For me, this situation is literally reversed. With Optimus turned Off in BIOS With Optimus turned On in BIOS So, no matter what I do, USB-C/DP is always driven by Iris, and HDMI is always driven by RTX. I have updated all drives from Dell website. I've updated the BIOS firmware. I've updated GeForce drives. In the pictures below, VZ279 is an HDMI-only monitor (connected to my HDMI output) and Dell U2913WM is what I have connected over the USB-C/DP output)   [IMAGE]   [IMAGE]   [IMAGE]   [IMAGE]   Furthermore, as I said, I've disabled Optimus in BIOS, and yet I still get output driven by Intel Iris. How is this even possible? Do I have a defective Laptop? It's still under warranty, should I replace it?  
G15-5520 USB-C/DP output driven by Iris instead of RTX 3060