Server Side dataTable dropdown filter only showing data in first page | PHP, SQL SERVER

This is a working script, but the dropdown option only displays the first 10 data and not all data in the table. What seems to be the problem here?

$(function() {
    $('#example').DataTable( {
        dom: "Bfrtip",
        ajax: {
            url: "account_type_table.php",
            type: "POST"
        },
        serverSide: true,
        columns: [
            { data: "seriesno" },
            { data: "accounttype" },
            { "data": "seriesno", "name": " ", "autoWidth": true, "render": function (data, type, full, meta) {
                return "<button class='btn btn-success btn-sm btn-flat edit' data-id='"+full.seriesno+"'><i class='fa fa-edit'></i> Edit</button> <button class='btn btn-danger btn-sm btn-flat delete' data-id='"+full.seriesno+"'><i class='fa fa-trash'></i> Delete</button>";}
                        }
        ],
        initComplete: function () {
        this.api().columns([0,1]).every( function () {
            var column = this;
            var select = $('<select class="form-control"><option value=""></option></select>')
                .appendTo( $(column.footer()).empty() )
                .on( 'change', function () {
                    var val = $(this).val();
                    column.search( this.value ).draw();
                } );

            console.log(column.data().unique());

            column.data().unique().sort().each( function ( d, j ) {
                select.append( '<option value="'+d+'">'+d+'</option>' )
            } );
        } );
    },
        select: false,
        buttons: [],
    } );
} );