Thanks for your reply.
I retried today making FW upgrade. This time I passed the Bootstrap Server (WSDI=23,1 where previous trials it returned WSDI=23,0).
but when I try to upgrade my FW from the AirVantage website, it neither fails or get stuck - see attached screenshot.
I also tried to reboot the device via the AirVantage website and it worked OK! --> device have communication with the DM server.
I noticed that the upgrade files are DELTA files from 3.4.4 or 3.4.3 FWs, where my FW is 2.5.1 --> I guess this is the reason I cannot upgrade. there are no other upgrade packages to install in the website…