Your AMD video card will probably work flawlessly out of the box without installing any additional drivers in Ubuntu. However if you are unlucky like me and experience random crashes, you might want to give the official drivers a try.

You can download the driver for your video card on the AMD website by selecting the exact model here: https://www.amd.com/en/support

Installing is quite straightforward, you just have to uncompress and run the included shell script for either the amdgpu or amdgpu-pro driver.

If your computer cannot boot into the graphical user interface, you can log in as root in the terminal and run the included uninstall script. All this is described in detail here: https://www.amd.com/en/support/kb/faq/gpu-635

What else can go wrong? Well the driver needs to build a kernel module and if your kernel changes, things can get complicated. In my case the resolution was stuck at 1024×768 pixels and I had no other options available.

I had a clue as the system stated that modeset was not enabled, so after some googling I found what I needed to set in grub on this page: https://askubuntu.com/questions/1094443/ubuntu-18-04-1-lts-r9-390x-amdgpu-guide-testing-summary

Edit the /etc/default/grub file:

sudo mcedit /etc/default/grub

Add these lines:

GRUB_GFXMODE=1920x1080x32
GRUB_CMDLINE_LINUX_DEFAULT="radeon.si_support=0 radeon.cik_support=0 amdgpu.si_support=1 amdgpu.cik_support=1 amdgpu.dc=1 amdgpu.dpm=1 amdgpu.modeset=1"

Run these commands to modify grub and reboot:

sudo update-grub2 && sudo update-initramfs -u -k all
sudo reboot