Some system designs tie specific outputs to one or the other GPU.  It's possible this one does -- or that the circuit requires the dGPU to complete the output.  You'll need to examine a schematic if you can find one, but in all candor, whoever did the repair did not think through the possible consequences.  You need a replacement system board.  
Hi, In my DELL G3 15 (3579) the motherboard and the dedicated nVidia graphics card were damaged.An independent service repaired the motherboard and removed the dedicated graphics card.(the integrated Intel UHD graphics card is enough for me for normal office work) Now the laptop works, but the HDMI port does not. I tried to connect an external Monitor or TV and it doesn't work. So, should the HDMI port work properly with the integrated graphics card (after removing the dedicated graphics card)?PS. To rule out problems with the Windows OS, I formatted the SSD drive and reinstalled the system from scratch, including drivers, but still not working
(G3 15 3579) The HDMI port doesn't work after removing the dedicated nVidia 
graphics