It's great to see you around @ZoeMuons. Could you please confirm that you have the One? Since, if your tracker doesn’t have GPS, you can still generate maps of your activity and capture other GPS data using the Fitbit app on your mobile device. This feature is called MobileRun.
To activate MobileRun from your iOS Fitbit app, you just need to do the following:
- First, make sure you can track your run, walk, or hike when you're out of Wi-Fi range:
- Open Settings and tap Cellular.
- Make sure Cellular Data is turned on.
- Next, ensure that your GPS can continue tracking when your phone is locked:
- Open Settings and tap General.
- Tap Background App Refresh, find the Fitbit app, and make sure it is turned on.
- Lastly, make sure your phone can identify your location:
- Open Settings and tap Privacy.
- Tap Location Services and make sure it’s turned on.
- Find Fitbit in the app list and make sure it's set to While Using the App or Always. If you choose the Always setting, MobileRun will automatically restart when unexpectedly interrupted.
I hope this helps, let me know the outcome. 
Alejandra | Community Moderator, FitbitIf you like something I recommended, I encourage you to mark that reply as "Best Answer". 🙂