I'm new to OpenWrt and I'm trying to test a custom firmware build. AFAIK I'm supposed to use tftboot. I soldered in a TTL serial to USB cable. If connect I the USB cable and then turn on the router, it acts dead and won't start. If I turn the router on and wait 2 seconds before hooking in the USB…

It means that the UART is running directly at SOC voltage, at 1.8 volts - not the more customary 3.3 volts. In case you want to connect to the serial console, you must use an adapter (or level shifter) operating at 1.8 volts. E.g. several (higher priced-) FTDI FT232RL (not all!) USB2serial adapters…

How to tftboot with Xiaomi AX3600?

Installing and Using OpenWrt

Gingernut January 12, 2021, 6:06am 6

Do you have a UART connection to the router working?

There is a recovery method that might work to send a custom firmware to the router.