Hi @jj14
mG are thousandths of G, local gravity. 1g = 9.8 m/s².
At some position one of the x, y or z axes has to be aligned with the local gravity, depending on the orientation of the sensor and will measure about +/- 1000 mG.
If no axis measures about 1000 mG, it is either in positions intermediate to the local vertical or it does not measure acceleration correctly.
The device may not have been configured correctly.
Try to uninstall a reinstall device