I am submitting this LuCI protocol that works with Aleksander0m’s ModemManager-openwrt package. It is able to set up and configure basic options of a modem.
Update: ModemManager-openwrt is now located here: https://gitlab.freedesktop.org/mobile-broadband/mobile-broadband-openwrt
Update ModemManager pull request here: https://github.com/openwrt/packages/pull/9137
Source code here:
Update - also here: https://github.com/nickberry17/luci-proto-modemmanager