I don't know if you found already the solution.
I got myself an M4800 whose previous user replaced the HD and installed Win10Pro not activated. I have been fighting with it to make everything work.
Right now I'm stuck at a similar point as you: My system runs on the embedded Intel HD4600 graphics adapter. Probably this is the reason the HDMI is not working (known issue when buying).
No device is detected in the Device Manager, and Nvidia driver installer fails with no compatible graphics hardware found
So I opened the laptop to find that there actually was an Nvidia board there (K2100M I suppose). I reseated the board and applied more thermal paste and closed again. with no luck.
BIOS is A25. The switchable graphics is ON.
OTOH, I'm a bit sad with the support in this Dell community, it's very common to find threads without any answer.