I have done a clean install and there is no QMI, and its still not working, I am experiencing the same problem as here: IoT hat disconnecting and reconnecting rapidly - #5 by zskatona
This seems to be a serious problem of the hardware, because I am using a 40 W Raspberry Pi 4 power supply, with no other peripherals, so the modem should have more than enough power available. I am using the short USB cable that comes with, I tried all USB ports, and it always keeps disconnecting and reconnecting.
This is a serious hardware bug and needs to be resolved ASAP Sixfab; this is unacceptable… I purchased two of these HATs for $125 each, only for none of them to work…