Geeetech Yun Shield working good... except bridge

Hello All, After spending hours trying to solve my issue, I finally (try to) see here if someone has an idea about this :

I have an UNO board and on top I plugged the GEEETECH YUN SHIELD.

The UNO is powered via 12V supply.
I can log on with ssh to the yun ->ok
I configured in boards.txt file the adequate stuffs, so the IDE find wifi board and is able to upload a sketch via wifi on the yun. The sketch is running good. I can play with leds.
So the SPI is working. 8)

Nevertheless, everything using the UART fails.
The jumpers on the YUN is configured on 5V (not 3V), and a jumper on the UNO disable the onboard usb/serial (I checked ...the UNO USB is no more working).

I have checked that when trying to use sketch using bridge() ... the blocking things is bridge.begin() on the UNO (this is documented as a blocking function).
So i concluded this function waits something that never happens on the YUN side.

I am wondering what to do next to troubleshoot this issue. Does anyone have an idea ?

Off course, The YUN is well configured as documented with

ARDUINO BOARD TYPE - "Uno with at mega328" and not something else

OPERATION MODE - Arduino Bridge

Since Geeetech is just rebrand/relable of Dragino:

Update Dragino Firmware

Expand the storage by USB flash drive and hard drive for Dragino:

Start easy, then transfer to Uno.

Basic Dragino setup.

The problem is that some of the Mega2560 (the ones bought on china) have the CH340G chip to control USB interface. This chip is quiet different from the original and doesnt have a reset pin (no jumpers), hence the USB connection get in conflict with the RX/TX pins used to communicate with the yun shield. To solve the problem is only need to remove the resistors 12 and 13 from the board. You will not be able to load a sketch by USB but works perfectly doing wireless .