I'm using Debian Buster, XFCE 4.12 as my DE, with an AMD RX 580 graphics card.

I have my primary monitor plugged on an HDMI port, and I wanted to use a secondary monitor on the DVI port, but this one is not detected.

At startup, the boot text is mirrored on both monitors, but at the lightdm greeter, only my primary monitor is on.

When I go to Settings > Display, only my primary monitor is detected

Only that display is detected by xrandr as well.

Additional info

lshw -c video

outputs

  *-display UNCLAIMED       
       description: VGA compatible controller
       product: Polaris 20 XL [Radeon RX 580 2048SP]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:06:00.0
       version: ef
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list
       configuration: latency=0
       resources: memory:e0000000-efffffff memory:f0000000-f01fffff ioport:e000(size=256) memory:fe900000-fe93ffff memory:c0000-dffff

And for xrandr

xrandr

outputs

xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 1920 x 1080, maximum 1920 x 1080
default connected 1920x1080+0+0 0mm x 0mm
   1920x1080      0.00* 
   1440x900       0.00  
   640x480        0.00  

And info about the OS and the kernel

uname -a

outputs

Linux debian-prout 4.19.0-5-amd64 #1 SMP Debian 4.19.37-5 (2019-06-19) x86_64 GNU/Linux

I suspect it is a problem with my drivers, but I can't figure out how to install/update them properly yet. I can see some text related to amdgpu firmware when I boot, but it's too fast I can't catch it.

What should I fix?