1.
copy with scp/winscp UAP-DD-WRT.bin to 192.168.1.20:/tmp
2.
login with ssh/putty to 192.168.1.20 and change directory to /tmp (cd /tmp)
3.
execute the following command
fwupdade.real -m UAP-DD-WRT.bin -d
Maybe you could try the initial install way, i.e. flashing with tftp:
set your pc static ip 192.168.1.x/24 (with x different from 20 and anything in the network you may be connected to).
Eventually insert a switch between the PC and AP so that the link won't go down on AP next reset and the pc network interface isn't reset to auto dhcp.
In the pc, cd to the folder where you downloaded the "Unifi AP Firmware - Console image for first installation" from Routers Database.
press reset button until leds blink
then
Code:
tftp 192.168.1.20
bin
put UAP-DD-WRT.bin
# and in the end
exit
Please wait confirmation from an expert as I don't know Unify specifics at all (only legacy Ubiquity NS/PS 2/5). I don't even know if there are leds.