New Machine with AMD 7nm on Ubuntu 18.04
There are a few things you need to do in order to get things up and running with the Navi GPU:
1.) Upgrade to the 5.4 kernel: by for instance installing the Ukuu kernel manager. Another popular manager is the mainline manager.
2.) Do not choose something newer than 5.4, like 5.4.2 or whatever, choosing one of those bricked my machine and I had to reinstall Ubuntu.
3.) At this point you will get proper resolution etc. with your Radeon 5700 RX card but this will for some unknown reason break Rhythmbox. Trying to launch Rhythmbox will crash your Xorg session and you’ll end up at the gdm or lightdm screen without any crash logs you can use whatsoever.
4.) Download and install the AMD binary GPU driver: just follow the instructions here.
5.) Done, you have now istalled the binary driver, you can now apt install rhythmbox and restart your machine and all is good.