Hi @tchan
When I read your issue, my first thought was the modem might be overheating and shuts down as a failsafe.
Now image issue is still possible but if it's the same with android and with Ubuntu Touch and you did the flashing multiple time then I guess the hardware issue is the more probable.
To test my hypothesis you can try to download a relatively large file in 2G, 3G and 4G... as I suspect maybe not all are being handled the same.