It may be that this is an incompatibility issue with the built-in Dell COM port and VirtualBox (that's what I got from the link @parky posted).
If so, then you could spend a long time shaving that yak, trying to sort it out.
A simpler solution might be to buy, beg, borrrow or steal a USB-serial dongle, and configure the VirtualBox to "own" that USB device, so the Host Dell Windows 10 OS will not even see the USB device when the VM is running. Then the Guest VM will be able to use the USB device as if it was directly connected to the VM, and the VM would then configure the USB serial device for its own use.
Not an ideal solution, but I suspect OP primarily wants a working COM port, and knowing how to make VirtualBox work with this Dell built-in COM port is irrelevant once they have a COM port that works.
I had a similar situation with a built-in network adapter on a laptop that would not work for me in a VirtualBox VM; I obtained a USB-ethernet dongle and assigned it to the VM; problem solved.