c++ – Problem occured in writting CMakeLists about include_directories and add_subdirectory on add_library

I am new to CMake project and
I am trying to write a simple test with libmodbus and paho-mqtt
on VSCode with GCC 8.1.0 x86_64-w64-mingw32.

In my main.cpp both head file can be included normally,

#include "modbus.h"
#include "MQTTClient.h"

int main(int argc, char *argv[]){
    return 0;
}

and CMakeLists.txt like this

cmake_minimum_required(VERSION 3.5.0)
project(Project_Testbed VERSION 0.1.0 LANGUAGES C CXX)

add_executable(${PROJECT_NAME} main.cpp)

include_directories(ext/libmodbus)
target_link_directories(${PROJECT_NAME} PUBLIC ext/libmodbus)
target_link_libraries(${PROJECT_NAME} PUBLIC modbus)
add_custom_command(TARGET ${PROJECT_NAME} POST_BUILD
    COMMAND ${CMAKE_COMMAND} -E copy
    ${CMAKE_CURRENT_SOURCE_DIR}/ext/libmodbus/modbus.dll
    $<TARGET_FILE_DIR:${PROJECT_NAME}>)

add_subdirectory(model/model_mqtt)
include_directories(model/model_mqtt)
target_link_libraries(${PROJECT_NAME} PUBLIC model_mqtt)

include_directories(ext/mqtt)
target_link_directories(${PROJECT_NAME} PUBLIC ext/mqtt)
target_link_libraries(${PROJECT_NAME} PUBLIC paho-mqtt3a)
add_custom_command(TARGET ${PROJECT_NAME} POST_BUILD
    COMMAND ${CMAKE_COMMAND} -E copy
    ${CMAKE_CURRENT_SOURCE_DIR}/ext/mqtt/paho-mqtt3a.dll
    $<TARGET_FILE_DIR:${PROJECT_NAME}>)

But when I try to do it in subproject(model_mqtt.cpp), like this

#include "modbus.h"
#include "MQTTClient.h"


int test(int argc, char *argv[]){
    return 0;
}

and CMakeLists.txt like this

project(model_mqtt)

set(mqtt_files
    model_mqtt.cpp
    model_mqtt.h
    )
add_library(model_mqtt ${mqtt_files})

target_link_libraries(${PROJECT_NAME} PUBLIC modbus)
target_link_libraries(${PROJECT_NAME} PUBLIC paho-mqtt3a)

It shows: cannot open source file “MQTTClient.h”C/C++(1696)

How could it be? Isn’t them the same?

My project structure is like this

project
|──ext
|   |──mqtt
|   |    MQTTClient.h
|   |    paho-mqtt3a.lib
|   |    paho-mqtt3a.dll
|   |    etc...
|   |──libmodbus
|   |    modbus.h
|   |    modbus.lib
|   |    modbus.dll
|   |    etc...
|──model
|   |──model_mqtt
|   |    CMakeLists.txt
|   |    model_mqtt.cpp
|   |    model_mqtt.h
|──CMakeLists.txt
|──main.cpp
|──main.h