Modem Firmware Upgrade - Modem Busy - USB Issues

[biketool]

I'm nut sure if lsusb lists the (sub-)functions of the USB device.
The modem knows the following tty:

Code:

mmcli -m any --output-keyvalue | grep -F modem.generic.ports.value
modem.generic.ports.value[1]                    : cdc-wdm0 (qmi)
modem.generic.ports.value[2]                    : ttyUSB0 (qcdm)
modem.generic.ports.value[3]                    : ttyUSB1 (gps)
modem.generic.ports.value[4]                    : ttyUSB2 (at)
modem.generic.ports.value[5]                    : wwan0 (net)

The kernel knows the following tty:

Code:

tree /dev/serial/by-id/
/dev/serial/by-id/
├── usb-Quectel__Incorporated_LTE_Module_community_fw-if00-port0 -> ../../ttyUSB0
├── usb-Quectel__Incorporated_LTE_Module_community_fw-if01-port0 -> ../../ttyUSB1
├── usb-Quectel__Incorporated_LTE_Module_community_fw-if02-port0 -> ../../ttyUSB2
└── usb-Quectel__Incorporated_LTE_Module_community_fw-if03-port0 -> ../../ttyUSB3

got it; I also show same from mmcli -m any --output-keyvalue | grep -F modem.generic.ports.value
I still don't know why AT commands get refused

Code:

mobian@mobian:~$ mmcli -m any --output-keyvalue | grep -F modem.generic.ports.value
modem.generic.ports.value[1]                    : cdc-wdm0 (qmi)
modem.generic.ports.value[2]                    : ttyUSB0 (qcdm)
modem.generic.ports.value[3]                    : ttyUSB1 (gps)
modem.generic.ports.value[4]                    : ttyUSB2 (at)
modem.generic.ports.value[5]                    : ttyUSB3 (at)
modem.generic.ports.value[6]                    : wwan0 (net)

For what it is worth, I also tried sudo echo -ne "AT+QFASTBOOT\r" > /dev/ttyUSB2 to try to get fastboot mode, same answer: Device or resource busy