

It doesn't need to be Debian you can use any Linux distribution.

Resize the OpenWRT partition to use entire disk space.Flash the SSD drive with the OpenWRT image.download OpenWRT image onto the Debian USB.Connect your Debian USB to APU board and boot.If you have an APU2, APU3, APU4 or APU6 router and don't want to go through the manual installation procedure, you can use the automatic OpenWRT installer. OpenWRT needs to be "flashed" on the disk.

OpenWRT doesn't have a typical installation process like pfSense or IPFire. Instructions for older and newer versions are almost the same, but some packages (linked at the end) may be different. OpenWRT installation instructions for APU2/APU3/APU4 boards
