Hi @BecauseScience,
The TX power you set in the advertising data should be the same as the actual TX power the device currently applying. The BLE.txPower(int8_t* txPower) API will return the actual TX power for you. And then you can call the advData.append(BleAdvertisingDataType::TX_POWER_LEVEL, ...) to insert it to the BLE advertising data object and call BLE.setAdvertisingData() to update the advertising data.
Best regards,
Guohui