You will need a UART 3.3V USB adapter, open the device and connect the UART to a PC and to it.
The UART pinout and some more instructions you can find here. I was able to make a successful connection using a three pin connector without soldering.
After that, you will need to run a serial terminal on the PC (Putty for example) connected to the UART serial port @115200 bps.
Turn on your device, while it is booting in the serial console press "x". This will activate the web recovery. Then open a browser at 192.168.0.1 and upload the factory firmware. Wait a few minutes and the router will be recovered with the factory firmware. I have not tried but it may be possible to recover directly with the OpenWRT factory image.
I believe (just an hypothesis) the web recovery is not automatically activated because uboot finds a valid kernel and tries to load it. And what happens is what it seems kernel gets stuck for some reason and does not report any error.
That's why the device ends up bricked without any activity in the ethernet ports because kernel did not load and web recovery is not activated. TFTP is also not an option.
Therefore the only option is to attach a UART and activate web recovery manually via serial console before kernel load.
That's correct. What I was referring is that some UART adapters have a jumper to configure 3.3V or 5V as the signal/TTL level (see below). It needs to be 3.3V otherwise if you use 5V it may/will damage your router.
The wires that need to be connected to the router are only three: GND, TXD, RXD.