Your posture has to be assessed properly before using a belt. We have to find out whether your posture is rigid or flexible and give corrections appropriately. The cause can be weakness of structures or tight structures or a combination of both. Sometimes it can be because of bony alignment.
Next Steps
Weak structures must be strengthened and tight structures should be stretched to improve posture. if the cause is bony alignment, much cannot be done
Health Tips
While a belt may temporarily improve posture, using it long term will cause weakness of the the muscles which will worsen your posture.