Tenda 3G150B unbrick no RX on UART

I bricked my 3G150B with a 3G150M firmware (yes, i was a lame, I know)
I followed the tips/tricks/instructions at the "Inside 3G150B" archived topic.
The serial communication is all right when I connect the TX and ground only (last 2 headers), the terminal goes through the U-Boot message, the operation choice and goes on with the default choice 3 (Boot system code via Flash).
However when I connect RX, no boot message at all on the terminal.
I tried to touch the RX header after the boot started but I couldn't send any keyboard command (eg. ctrl+u or 2) this way
I use Raspberry Pi minicom as serial terminal via the UART pins (RPi TXD - GPIO 14/8 to Tenda RX, RPi RX - GPIO 15/10 to Tenda TX and the grounds). The RPi serial works all right with other devices like IP Cameras.
Any idea what may cause the fail when I connect the RX header on Tenda?