R7 and RX series is not likely working due to not supporting legacy Bios and MSDOS VESA Video mode 103.  This mode is default due to VGA support for monitors from 1981 and up.

Graphics modes

320×200

640×400

640×480

800×600

1024×768

1280×1024

256-color palette

15-bit (5:5:5)

16-bit (5:6:5)

24-bit (8:8:8)

Stock power supply in 390 790 990 is LESS THAN 300W and therefore 75W cards are not supported.  390 790 990 is a bios system that requires MSDOS VESA video mode 103 for the F2 CMOS Setup.  RX Series cards do not support legacy MSDOS or VESA or XP and require UEFI to initialize.   Non Dell OEM cards require Secure boot OFF and CSM ON. 

When installing an after-market graphics card into a Windows PC with UEFI secure boot enabled, the system may not boot.  This is true for ANY PC not just Dell.

http://nvidia.custhelp.com/app/answers/detail/a_id/3156

  There is no UEFI or CSM in a 390 790 990 so from both hardware and software its not ever working.

Recommended card for Older Dells up to current is GT1030.

https://www.youtube.com/watch?v=2y32mBdZHJY

Works in all models EXCEPT USFF and Micro because they do not have slots or 75W PCI-E slot power.

The Dell GT1030 for Tower models is 8CCF1.  The Retail parts are also available from Dell.

I'm using a low profile. AMD Radeon R7 250 2G