Disable secure boot prior installing a new GPU or buy GPU from dell supported on this model.  Also do a clean install of operating system using Microsoft image which will not have any customization from dell. You can download the image and make a bootable USB using Media creation tool 
I've just got a Dell 8940, but it's having a lot of issues, for example Support Assist will not install, when I try to install it was it needs to install .net, when I say Yes go ahead, it does nothing. Another issue is I often get the fatal error "Reference by pointer". When I try to use recovery, that starts, then it says internet is not working (even though when the PC is up internet works fine). Other issues I've had is Microsoft Store stopped working. ~The PC is driving me nuts. Hardware scans report no error.  I've had the PC for about a week.
Support Assist