trying to send bytes from peripheral to central (DA14580 to smartphone)

I am trying to send a n array of bytes from the BLE chip to the smartphone, AFTER I receive an array of bytes in the BLE chip from the smartphone, that is, my intended order of operation is as follows:

Smartphone --BLE--> DA14580 chip --BLE--> smartphone

I have managed to do the first part (Smartphone --BLE--> DA14580 chip) but I am having trouble understanding how to do the second part

I am modifying the ble_app_peripheral sample program

I took the code inside void app_adcval1_timer_cb_handler() in user_custs1_impl.c, and tried to modify it, but it doesn't work (that is, my bustom BLE receive app on my smartphone does not detect anything)
(I am also sure it is not a problem with the smartphone app, as it can detect incoming BLE data from another custom BLE module that I have, and my goal to create my own version of this custom module)

Here is my code:

void user_server_rx_data_ind_handler(ke_msg_id_t const msgid,
struct custs1_val_write_ind const *param,
ke_task_id_t const dest_id,
ke_task_id_t const src_id)
{
arch_printf("in function %s\n\r", __func__ );
//DO OPERATION: just display data for now
for(int i=0; ilength; i++){
arch_printf("\tparam->value[%d] = %d (0x%x)\n\r", i, param->value[i], param->value[i] );
}

//2018-02-12: I2C send byte to external dev
//send the data part of the received byte array to I2C slave address
arch_printf("Writing to I2C: Add: %d value: %d\n\r", I2C_SLAVE_ADDRESS, param->value[(param->length) - 1] );
i2c_write_mine(I2C_SLAVE_ADDRESS, param->value[(param->length) - 1]);

//2018-02-20: from forum post (https://support.dialog-semiconductor.com/ble-transmission)
//*************************************Custom value 1*********************************//
struct custs1_val_ntf_req* req = KE_MSG_ALLOC_DYN(CUSTS1_VAL_NTF_REQ,
TASK_CUSTS1,
TASK_APP,
custs1_val_ntf_req,
DEF_USER_SERVER_TX_DATA_CHAR_LEN);

arch_printf("Inside %s\n\r\tCUSTS1_VAL_NTF_REQ = %d\n\r\tTASK_CUSTS1 = %d\n\r\tTASK_APP = %d\n\r\tDEF_CUST1_ADC_VAL_1_CHAR_LEN = %d\n\r",__func__, CUSTS1_VAL_NTF_REQ, TASK_CUSTS1, TASK_APP, DEF_USER_SERVER_TX_DATA_CHAR_LEN);
static uint16_t sample[20];
for(int i=0; i< 20; i++)
{
sample[i] = 65 + i;
}

req->conhdl = app_env->conhdl;
//req->handle = CUST1_IDX_ADC_VAL_1_VAL; // 2018-02-20 : ORIG
req->handle = USER_IDX_SERVER_TX_DATA_VAL;
//req->length = DEF_CUST1_ADC_VAL_1_CHAR_LEN; // 2018-02-20 : ORIG
req->length = DEF_USER_SERVER_TX_DATA_CHAR_LEN;
//memcpy(req->value, &sample, DEF_CUST1_ADC_VAL_1_CHAR_LEN); // 2018-02-20 : ORIG
memcpy(req->value, &sample, DEF_USER_SERVER_TX_DATA_CHAR_LEN);

ke_msg_send(req);

arch_printf("I2C data sending completed\n\r");
}

user_server_rx_data_ind_handler() is the custom function that gets called when DA14580 receives bytes from BLE (this function is called inside user_catch_rest_hndl() in user_peripheral.c, inside one of the switch()...case blocks

USER_IDX_SERVER_TX_DATA_VAL is defined in user_custs1_def.h, as an enum along side CUST1_IDX_ADC_VAL_1_VAL. This enum entry was created following the tutorial for the ble_peripheral_app. The other enums created alongside this are:

USER_IDX_SERVER_TX_DATA_CHAR,
USER_IDX_SERVER_TX_DATA_VAL,
USER_IDX_SERVER_TX_DATA_NTF_CFG,
USER_IDX_SERVER_TX_DATA_USER_DESC,
Basically I have tried to make this the same as CUST1_IDX_ADC_VAL_1_*

DEF_USER_SERVER_TX_DATA_CHAR_LEN is defined in user_custs1_def.h as
#define DEF_USER_SERVER_TX_DATA_CHAR_LEN 20

The ble_peripheral_app tutorial I followed is
Training_03_custom_profile_gatt_cmd_example_v1 0.pdf

I think I am missing something very basic here. How can I send a single byte of data from the BLE chip to the smartphone?

--- EDIT ---
This causes the following outputs on the UART terminal:

Inside user_server_rx_data_ind_handler
CUSTS1_VAL_NTF_REQ = 55300
TASK_CUSTS1 = 54
TASK_APP = 50
DEF_CUST1_ADC_VAL_1_CHAR_LEN = 20
I2C data sending completed

It also causes a call to the user_catch_rest_hndl () function in user_peripheral.c

Entered user_catch_rest_hndl :
msgid = 0xd805
CUSTS1_VAL_NTF_CFM detected : msg_param->handle = 27

--- EDIT 2 ---
The following entries were also added to the user_custs1_def.c file's struct attm_desc_128 custs1_att_db[CUST1_IDX_NB] = structure definition:

//user Server TX data Characteristic Declaration
[USER_IDX_SERVER_TX_DATA_CHAR] = {(uint8_t*)&att_decl_char, ATT_UUID_16_LEN, PERM(RD, ENABLE),
sizeof(user_server_tx_data_char), sizeof(user_server_tx_data_char), (uint8_t*)&user_server_tx_data_char},

//user Server TX data Characteristic Value
//might have to enable NTF permission here; come back to this point if needed later
[USER_IDX_SERVER_TX_DATA_VAL] = {USER_SERVER_TX_DATA_UUID_128, ATT_UUID_128_LEN, PERM(RD, ENABLE) | PERM(NTF, ENABLE),
DEF_USER_SERVER_TX_DATA_CHAR_LEN, 0, NULL},

//user Server TX data Characteristic Configuration Descriptor
[USER_IDX_SERVER_TX_DATA_NTF_CFG] = {(uint8_t*)&att_decl_cfg, ATT_UUID_16_LEN, PERM(RD, ENABLE) | PERM(WR, ENABLE),
sizeof(uint16_t), 0, NULL},

//user Server TX data Characteristic User Description
[USER_IDX_SERVER_TX_DATA_USER_DESC] = {(uint8_t*)&att_decl_user_desc, ATT_UUID_16_LEN, PERM(RD, ENABLE),
sizeof(USER_SERVER_TX_DATA_USER_DESC) - 1, sizeof(USER_SERVER_TX_DATA_USER_DESC) - 1, USER_SERVER_TX_DATA_USER_DESC},