If you did end up with an open U-Boot, I saw output immediately when I connected my UART<->USB dongle and opened putty.
All settings were standard with baud 115200 if I remember correctly. Referring to the image back when I attempted the repair this is what the UART pins need to connect to
I physically opened the router backlid for easier access but then just held the jumper cables for the UART connection by hand (no solder).
I think if you have a copy of the mi WiFi recovery tool, it's probably easier to just redo the process from the recovered image 