Make sure you have installed armbian-firmware-full. The default builds only include a trimmed down set of firmware that is intended to cover the common ones, but to save space doesn't include all the firmwares available in mainline linux kernel. That may solve your problem.
If you haven't already, I would suggest you open the box and see what wifi/bluetooth chip you have on your board. Then you need to make the dtb file match what hardware you have.