RAK811 TrackerBoard do not respond to AT commands

Hi everyone,

In order to complete a project, I need to connect a device to my gateway (which I already configure).
The problem is that after burning the firmware, the module do not respond at all on my serial port… Before the burn, the device did not respond to my at commands, but at least it sent me gps datas…
I did the firmware flashing on boot mode, and now no matter if I’m on boot mode or normal mode it doesn’t work!
I don’t think it’s a big deal but I can’t currently find the solution… Thank for your help guys if you can…