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Unfortunately, that phone isn't on fitbit's list of supported devices at www.fitbit.com/devices and I have found a number of other posts where people have reported problems using it.
There is a feature suggestion asking fitbit to resolve these issues and the phone. You may want to add your vote: https://community.fitbit.com/t5/Feature-Suggestions/LG-Stylo-4-compatibility/idi-p/2864865
In the meantime, do you have access to a computer, tablet, or other phone that you could use?
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