The problem is that the documentation says to upgrade the “kernel to version 4.4.143-41-rockchip or newer.” but when I run “sudo apt-get install -y linux-4.4-rockpis-latest” I get a reply that “linux-4.4-rockpis-latest is already the newest version (4.4.143-39-rockchip)”.
So I switched to the testing repository, upgraded, rebooted and it worked:
# deb http://apt.radxa.com/buster-stable/ buster main
deb http://apt.radxa.com/buster-testing/ buster main
This was bugging me for a bit until I saw that it needed the latest kernel.