How to restart the modem on Arch

[kqlnut]

Sometimes I have to run my script 2 times.
I wonder if systemd would even start a self-made script using your approach; if so + if I find out how to improve the script I would try your approach + my script

Yes, you can start any script on the ExecStart line. If you find an indicator for a non working modem that can be checked by code, you could for example also have the script check it, restart ModemManager, check again a couple of seconds later (however long it usually takes to get it back up) and if necessary restart it a second time.