Flustered is a client library only, and cannot emulate a Bluetooth device in peripheral mode. It has a companion package https://github.com/Polidea/blemulator_flutter but that is only for testing purposes (faking a simulated device).
https://github.com/juliansteenbakker/flutter_ble_peripheral however is truly a plugin developed for implementing and advertising a BLE device with services and characteristics. Keep in mind that iOS is more restrictive about what you can do.
We need more details than just "But not working...". What code have you written so far and what precise and exact errors you faced?