I'm not sure this is of any value, but I recently purchased a used 2017 C-Max Energi SE. I still have the 3G modem and had tried for several days to connect the car to MFM to no avail.
I downloaded the app, created an account, registered my car in the app (using the VIN), and then turning on the car. According to Ford, a pop up should show up on the car screen asking to allow it to sync to the app. This didn't happen. I tried resetting sync from the screen and then deleting and re-adding the car in the app, the pop never appeared.
I just figured I would need the 4G upgrade and let it go at that but continued to do some research. On some blog or website I found a suggestion with these steps:
Turn ignition off
remove fuse F67 located in the passenger fuse block for at least 3 minutes (this fuse powers the modem)
reinstall the fuse
delete the vehicle from MFM
re-add and activate the vehicle in MFM
start the car
Then the pop up asking to allow sync to MFM appeared!
Success! I am now able to start, lock, unlock, and see battery and fuel levels, location, etc.
At least until 3G stops functioning, but at least I know the functionality works.