If you are trying to use your Cannondale Wheel Sensor with both the Cannondale App and Garmin devices or the Garmin Connect app, there are several known problems to be aware of. See the articles linked here.
Garmin devices (Edge, Forerunner, Fenix, etc.)
When the Cannondale Wheel Sensor is paired with current model Garmin devices (Edge, Forerunner, Fenix, etc.) for rides, the Garmin device software makes the ride files on the sensor unavailable for later syncing with the Cannondale app. It will appear like you never did the ride(s) in your Cannondale App ride history. There is a work around. See this article for details: Using the Wheel Sensor with both the Cannondale App and Garmin devices
Garmin Connect app
Using the Cannondale Wheel Sensor with both the Cannondale App and the Garmin Connect mobile app for iOS or Android causes known problems. For reliability using the Cannondale App we recommend against pairing the Cannondale Wheel Sensor with the Garmin Connect app. See this article for details and how to remove the sensor from Garmin Connect: Using the Wheel Sensor with both the Cannondale App and the Garmin Connect app