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.
Documentation on how to use the snap can be found in https://docs.ubuntu.com/core/en/stacks/network/modem-manager/docs/.
Please find the source code at https://code.launchpad.net/~snappy-hwe-team/snappy-hwe-snaps/+git/modem-manager