How to install amd radeon r9 380 drivers if the nomodeset parameter is set, without which the graphics will not start?

Question

ubuntu

So... I'll start with where everyone starts.
I decided to switch to Linux (now I'm trying to sit on kubuntu). I'll go from the beginning.
The installation of the distribution did not go smoothly. GUI did not load. I quickly solved this problem by introducing the nomodeset parameter. After that, there were no more problems with the installation. Linux met me with the lack of a default driver. Of course, I went to the office. site, found my video card and downloaded the archive with the executable file, started installing and from there I stumbled over the first stone.
Installation stopped at 99% with this text:

WARNING: nomodeset detected in kernel parameters, amdgpu requires KMS

And why did I come here? How to finally solve this problem?! He shoveled everything he could shovel!
PS
I turned off the parameter. As a result, the graphics are not loaded. To be precise, on the screen: "Check the cable signal", which, as far as I know, indicates that the video card does not start (the coolers are spinning, it works with Windows 10). The fact that open source drivers don't work confirms the image resolution of 640 by 480 pixels, when with the same hardware on windows it was something like 1280 by 1024.
Oh yeah, I almost forgot! After an unsuccessful installation of drivers, applications installed by the installer begin to crash, starting from something unknown to me, ending with the console and browser.
PSS
Exhaust lspci -k | grep -EA3 'VGA|3D|Display':

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Tonga PRO [Radeon R9 285/380] (rev f1)
Subsystem: Micro-Star International Co., Ltd. [MSI] Tonga PRO [Radeon R9 285/380]
Kernel modules: amdgpu
01:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Tonga HDMI Audio [Radeon R9 285/380]

6 GB DDR3 RAM
Processor: amd athlon 2 x3 425 2.7GHz

Linux kernel (exhaust uname -r):

5.4.0-42-generic