GL-iNet E750: Unable to keep the modem powered down manually (via AT & /dev/ttyUSB2)

I am trying to force a modem power down by sending AT commands to the modem device interface. The modem powers down and the ttyUSB interfaces disappear briefly but then the modem is restarted somehow. I want the modem to stay powered down. How do I achieve this?

I have been sending: 'gl_modem AT ‘AT+QPOWD=0’ and ‘echo -e “AT+QPOWD=0” > /dev/ttyUSB2’

I receive a response from the modem that the device has powered down successfully. But then a few seconds later it is back on again.

Why is the modem not staying powered off?