If I'm not mistaken, you need a kernel whose version is equal or higher than 5.1. To install one, you can add Debian Backports to your list of repositories (follow the instructions there). Then, install the version that you wish.
To install the firmware of the Intel AX200 NGW, you can also use the same repository. You just need to upgrade the "firmware-iwlwifi" package. Use this command:
apt-get install --target-release=buster-backports firmware-iwlwifi
The advantage of using this repository, instead of copying the firmware by hand, is that you will get updates automatically.