GPS and Bluetooth on, any battery saving mode on phone must be turned off. on the watch start the bike workout, and you will see the icons in upper part of screen should be like below.. see the gps icon, and mobile icon
If this is the case, you are set to go for a nice bike riding. I myself got lot of inconsistency problems with connecting the fitbit with gps of mobile, so an alternative option would be to get the Strava app and sync your fitbit account, and log every bike ride with the strava app which will appear after syncing also in your fitbit account.
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