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As btta rightly said, the stack takes care of sending the MTU response when we receive the MTU request. The GATTS_REQ_TYPE_MTU event is just an iformative event and you need not call any API to send the MTU response.
By default, the MTU size defined in the wiced_bt_cfg_settings_t.gatt_cfg.max_mtu_size is used. If you wish to change the MTU size, then use this API: wiced_bt_gatt_configure_mtu().
In your case, the issue might be the attribute length. In your attribute_t table, check the length of your attribute. It might be set to 20. Increase this size to let the iOS app write more than 20 bytes at a time.