AMD has released a new Radeon RX 5700 Series driver for Ubuntu Linux 18.04 LTS

Radeon Software for Linux 19.30 for Radeon RX 5700 Series Highlights
Provides support for the Radeon RX 5700 Series of graphics cards

Known Issues
[AMDGPU-Pro] Dota 2 players might experience instability running the game under Vulkan
[AMDGPU-All Open] Under certain conditions UVD might not engage during 4K HEVC media playback
[AMDGPU-All Open] Corruption observed when resizing MPV Media player
[AMDGPU-All Open] Potential instability in some older OpenGL titles

  AMD Radeon Software for Linux 19.30 for Radeon RX 5700 Series