# First, mask NetworkManager (if not done already)
systemctl mask NetworkManager

# Unmask, enable and start ModemManager
systemctl unmask ModemManager
systemctl enable ModemManager
systemctl start ModemManager

# Add a .network file for your modem network interface:
cat /etc/systemd/network/wwp0s20u6c2i12.network:

[Match]
Name=wwp0s20u6c2i12
[Network]
DHCP=both

# restart systemd-networkd:
systemctl restart systemd-networkd


# Scripts to enable and disable the modem and the corresponding network
# interface:

cat /usr/local/bin/modem-connect:

#!/bin/sh
mmcli -m 0 --simple-connect="apn=APN_HERE"
# also enter ,pin= if needed
ifconfig wwp0s20u6c2i12 up


cat /usr/local/bin/modem-disconnect:

#!/bin/sh
ifconfig wwp0s20u6c2i12 down
mmcli -m 0 --simple-disconnect