Raspberry Pi + SIM800 for GPRS

[jpmeijers]

Seems like one need to enable/power-on the modem before the serial connection becomes available:

Toggle GPIO from host at startup

I have a LTE hat/shield for my Raspberry Pi. I want to use this for internet backhaul connectivity. ModemManager can use the hat, but to power on the hat I need to toggle GPIO18 low for 1 second. Only after powering on the hat I have a serial connection to talk to the modem, and only then ModemManager can detect and use the modem. Is there a way to install a simple systemd service on the host that will run before ModemManager starts up, and which can toggle the GPIO for me?