Spring Boot Pagination is not working with custom DTO list

Question

I am stuck with integrating pagination in Spring boot project.

Service Impl class

@Override
public Page<OnlineBuyProductReportItemDTO> generateOnlineBuyProductReport(String fromDate, String toDate, int pageNum, String sortField, String sortDir) {
    Pageable pageable = PageRequest.of(pageNum - 1, 10, 
            sortDir.equals("asc") ? Sort.by(sortField).ascending()
                                  : Sort.by(sortField).descending()
    );
    List<PurchaseOrder> purchaseOrderList = purchaseOrderRepository.searchPurchaseOrdersForOnlineBuyProductReport(
            DateUtil.convertToDate(fromDate), DateUtil.convertToDate(toDate));

    OnlineBuyProductReportItemDTO item = null;
    List<OnlineBuyProductReportItemDTO> itemList = new ArrayList<OnlineBuyProductReportItemDTO>();
    if(purchaseOrderList != null && purchaseOrderList.size() > 0) {
        for (PurchaseOrder purchaseOrder : purchaseOrderList) {
            
            item = new OnlineBuyProductReportItemDTO();
            item.setOrderNumber(purchaseOrder.getOrderNumber());
            item.setCreatedDate(DateUtil.convertDatetoStringSwissDate(purchaseOrder.getCreationTime()));
            
            Address address = addressRepository.getAddressById(purchaseOrder.getBillingAddressId());
            item.setCustomerName(address.getFullName());
            item.setContactNumber(address.getTelephone());
            
            List<OrderCart> orderCartList = orderCartRepository.getOrderCartByPurchaseOrderId(purchaseOrder.getId());
            List<String> buyProductNameList = new ArrayList<String>();
            if(orderCartList != null && orderCartList.size() > 0) {
                
                for (OrderCart orderCart : orderCartList) {
                    buyProductNameList.add(orderCart.getProductName());
                }
            }
            item.setProductName(buyProductNameList);
            item.setPrice(purchaseOrder.getTotalPrice());
            item.setStatus(purchaseOrder.getDeliveryStatus().getName());
            itemList.add(item);
        }
    }
    
    return new PageImpl<OnlineBuyProductReportItemDTO>(itemList, pageable,itemList.size());
    
}

Here I have fetch data from 3 database tables and after data processing I have added to these data to DTO List.

But, when after integrated with the front end, pagination options are displayed but did't work as expected. All the pages containing same data set and also sorting options are not working as expected.

It seems this pageable part is not applying to the data processing.

Pageable pageable = PageRequest.of(pageNum - 1, 10, 
            sortDir.equals("asc") ? Sort.by(sortField).ascending()
                                  : Sort.by(sortField).descending()
    );

Is there any way to do this