Hello,
I am also interested in adding support for this device. I have opened my device and managed to solder the UART pin headers to the PCB. There are some missing elements on the board that need to be bridged or else the headers won't work. In my case I've added a bridge across R902 and R912 while removing the resistor at R24. This seems to work for my 3.3V USB-to-serial adapter and I am able to connect to the device. Much to my surprise the device already seems to be running OpenWRT AA-rc1 as the stock firmware.
Here is the device stock boot log example: https://pastebin.com/ibQsPPDv
Here is some extra info I pulled off: https://pastebin.com/MrNw8PKA
