Serverside-Processing & AJAX - Select filter not works

Hi!

I need to implement filter by select input to my table. I using serverside-processing to get data from DB and AJAX for dynamic actions.
Pagination, entries and search works fine but only this filter not, why?
Here is my code:

$(document).ready(function() {
    table = $('#VolunteersList').DataTable({ 
        searching: true,
        "processing": true,
        "serverSide": true,
        "order": [], 
        "ajax": {
            "url": "<?php echo site_url('wolontariat/przegladaj')?>",
            "type": "POST"
        },
        "language": {
            "url": "<?php echo site_url('assets/js/dataTables.Polish-lang.json') ?>"
        },
        "lengthMenu": [[10, 25, 50, -1], [10, 25, 50, "Wszystko"]],

        "columnDefs": [
        { 
            "targets": [ -1, 0, -2 ],
            "orderable": false,
        },
        ],

        "fnRowCallback": function( nRow, aData, iDisplayIndex ) {
            nRow.className = aData[0 /* or whatever */] == "94" ? "HighlightUser" : "";
            // USTAWIĆ ZWRACANIE HIGHLIGHT Z BAZY
            return nRow;
        },

    });

});

    setInterval( function () {
        reload_table();
    }, 5000 );

    function reload_table() {
        table.ajax.reload(null,false);
    }

}