The C5 uses GPS two ways: "connected" AKA "Phone" GPS (meaning using the Phone's GPS) or "on device" AKA "Built-in" GPS (meaning just using the GPS built into the C5. There is also a "Dynamic" mode which will use Phone or Built-in depending on if the device can connect to the Phone at the time of the exercise.
There are two settings for this. First, you have to set the general GPS setting to one of the following 3 settings: "Phone", "Built-in" or "Dynamic". I would set it to "Dynamic" for now to see if GPS is working at all. Reach the GPS setting by swiping down from the time screen (on the C5) and swiping to "Settings." Tap on Settings, then swipe up to GPS. Tap GPS and then tap the GPS settings to toggle between "Phone", "Built-in" or "Dynamic." Stop on Dynamic.
Next, for each Exercise Mode, there is a GPS on/off setting. Make sure this is ON. On the C5, swipe from the time screen to the Exercise screen and select "Run" (for example.). Swipe up and make sure the GPS toggle for that exercise is turned "On."
To test, put your phone in airplane mode and start an exercise that has GPS enabled. You would see an indicator on the C5 screen that shows GPS is connected. After the exercise, sync with your phone and see if a GPS track was recorded in the App.
To test "connected" GPS, have airplane mode turned off and connect the phones GPS. On my phone I have to switch "Location Mode" from "Battery Saving" to "High Accuracy." Sync with the C% from the app and make sure you have a good sync before starting the exercise. If you have a good sync, start the exercise and when done, see if you have a good GPS track for it in the app.
Maybe there is something wrong with the GPS on your C5? Maybe try a device reset.
I have my C5 set to "Dynamic: and mostly use connected GPS because it saves the battery on the C5, but I have used both.
I would start with "on device" GPS.
