I see the same issue, i downloaded the latest OS available today on armbian https://dl.armbian.com/nanopim4/Buster_current
Seems after the install both wifi and bluetooth not working and no adapter is displayed.
this is the OS command i use and seems its a different board being listed in their image instead of RK3399
root@nanopim4:~# uname -a
Linux nanopim4 5.4.6-rockchip64 #19.11.4 SMP PREEMPT Fri Dec 27 00:45:52 CET 2019 aarch64 GNU/Linux
I tried replacing the mentioned firmware files as per @martinayotte and after a reboot it worked just fine.!!
Thanks
