ModemManager is a service which controls mobile broadband
ModemManager is a DBus-activated daemon which controls mobile broadband (2G/3G/4G) devices and connections. Whether built-in devices, USB dongles, bluetooth-paired telephones or professional RS232/USB devices with external power supplies, ModemManager is able to prepare and configure the modems and setup connections with them.
The modem-manager snap is usually used together with the network-manager snap. The snap is geared towards devices and IoT and is optimized for Ubuntu Core. Its usage on desktop/server Ubuntu is possible, but manual connection of interfaces is needed and do not expect full integration with the GUI.