Check the documentation for your modem and micro-controller.
The modem docs will most likely tell you to connect it to the computers serial port and then use the AT command set that it supports.
Does the micro-controller have an OS on it? If it does then it'll most likely be another serial port connection, if not then you'll have to build something yourself to run on the board.
This[
^] looks like it might be helpful for building something for it.
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