Hey there @sumitR., it's nice to see you around the forums! Thanks for letting me know you've already contacted our Support Team. It seems that you were told during the call that the Vivo X21 is not yet a supported device. Note that the devices on this list have been confirmed as fully compatible with all features of Fitbit devices and the Fitbit app. While a phone or tablet may not be verified as fully compatible, the Fitbit app may still work with your device.
Our team routinely test new devices to ensure full compatibility with Fitbit devices and the Fitbit app, and are constantly working to confirm compatibility with new phone models. Be sure to verify that your phone meets the minimum operating system requirements. Note that the majority of Android phones running version 4.4 or higher will work with our products.
If your phone supports Bluetooth 4.0, I'd recommend giving the setup flow a try to see if you can successfully connect your tracker.
Please keep in mind that when a device is not supported we cannot guarantee its functionality with the Fitbit app. It might work at a certain time and then it can stop working out of nowhere after an update. This is because the app is not optimized to that device and any change could impact the performance.
Let me know if there's anything else I may assist you with.
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