I’m the owner of a R9 390 that came with its own share of problems (pretty much the only distro that works is Linux mint 18.3) but after using these parameters" 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" and blacklisting Radeon I got it working temporarily.
The problem now is that at random moments the screen would go black the keyboard will be unresponsive and the only solution would be to do a hard restart.
Any tip will be greatly appreciated
Hi and welcome to the Forum 
Can you show the info on the card via;
/sbin/lspci -nnk | egrep -A3 "VGA|Display|3D"
I think you have a CIK card, but need the above to confirm, a lot of the options selected are not present anymore (eg dpm).