ModemManager
ModemManager is a DBus-activated daemon which manages cellular modems (2G, 3G, 4G and 5G) and establishes cellular data connections.[1]
Configuration
It is possible to enable the modem with:
networking.modemmanager.enable = true;
A good rule of thumb is to use NetworkManager as much as possible for WWAN functionality and rely on ModemManager functionality directly only when necessary (e.g., when having to insert a PIN to unlock the SIM card).