Changing the transmission power on ble_app_att_mtu_throughput example


I have searched Devzone for changing the transmission power; even though there were similar issues related to mine but at the end, It did not help me.

I am evaluating the new PHYs of BLE 5 in an indoor environment at the university.  

I tried to use the following codes:

ret_code_t err_code = sd_ble_gap_tx_power_set(BLE_GAP_TX_POWER_ROLE_SCAN_INIT, BLE_CONN_HANDLE_INVALID, RADIO_TX_POWER); ( inside scan_start function)

err_code = sd_ble_gap_tx_power_set(BLE_GAP_TX_POWER_ROLE_CONN, BLE_CONN_HANDLE_INVALID, RADIO_TX_POWER); ( I used this one inside 

ble_evt_handler function under BLE_GAP_EVT_CONNECTED.)

err_code = sd_ble_gap_tx_power_set(BLE_GAP_TX_POWER_ROLE_ADV, m_adv_handle, RADIO_TX_POWER); (I used this one inside advertising_start function)

I get fatal error!! 

I know there is a long range example but I really want to use the throughput example with modified transmission power. 

I do not know in which part I am doing wrong!!! Any help will be appreciated.