I tried to use fglrx, which isn't up to date anymore (I can't install it, but I allowed the repository thing in Ubuntu Software Center). After that I tried How to install AMD graphic drivers on Ubuntu 18.04 from Joshua and that didn't worked too.
AMDGPUPro isn't a solution, because it doesn't support the GPU I have.
Now I don't know what I am able to do, since the other answers are outdated.
