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Hey there @GurditSingh, thanks for stopping by! As you may have seen in the list that my friend @NellyG shared with you, the One Plus 5T is not a compatible device with the Fitbit app.
Which means that our Android team has not yet tested them for compatibility and added them to the list what means that we don't have a estimated date on when they will be added. If the device supports BLE (AKA Bluetooth 4.0), that would be an explanation of why you could be able to setup and sync your Ionic with the 5T. So, you can try the tips on the following help article: My Fitbit device isn't receiving notifications from my phone.
I'd like to mention, just to be sure, 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. The developer team is aware of the need to add new devices to the list of compatible devices. 
Hope the steps work. Don't hesitate to get back to me if you have any questions!
Best Answer
@MarreFitbit wrote:Hey there @GurditSingh, thanks for stopping by! As you may have seen in the list that my friend @NellyG shared with you, the One Plus 5T is not a compatible device with the Fitbit app.
Which means that our Android team has not yet tested them for compatibility and added them to the list what means that we don't have a estimated date on when they will be added. If the device supports BLE (AKA Bluetooth 4.0), that would be an explanation of why you could be able to setup and sync your Ionic with the 5T. So, you can try the tips on the following help article:
I'd like to mention, just to be sure, 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. The developer team is aware of the need to add new devices to the list of compatible devices.
Hope the steps work. Don't hesitate to get back to me if you have any questions!
Disabling battery optimization was a suggestion I came across elsewhere, specifically applicable to the OnePlus 5T. I understand that the list of approved phones that Fitbit puts out doesn't include the OnePlus 5T, but I'm pushing forward with it anyway. Other Fitbit trackers I've had in the past have worked with other OnePlus phones I've had. The list of Android phones that Fitbit has said work with their trackers doesn't seem well vetted, seems to just be a convenient way to dismiss issues when they arise.
Best Answer