jQuery Ajax scripts do not work on mobile browsers (any kind)

jQuery Ajax scripts do not work on mobile browsers (any kind). It works on any kind of desktop browsers and in a their simulations of mobile. But when uploaded on production server and tried to execute from any mobile phone, it does not work.

Edit: I've tried to post whole piece of code so some obvious errors could easily be detected.

let check_mobile = false;

<?php if (isMobile) { ?>
    check_mobile = true;
<?php } ?>

alert('ok');


$('.prod-img-wrap').each(function(ind) {

    let imgs_scope = {},
        front = $(this).find('.js-itm-hover-img-front'),
        back = $(this).find('.js-itm-hover-img-back'),
        img_front = front.attr('data-img'),
        img_back = back.attr('data-img'),
        id = front.attr('data-id');

    imgs_scope[id] = {
        'img_front': img_front,
        'img_back' : img_back ? img_back : null,
    };

    setTimeout(function(){

        $.ajax({

            url: 'index.php?route=product/category/product_thumb_work',
            type: 'get',
            data: { ids_imgs: imgs_scope },
            dataType: 'json',

            success: function(json) {


                front.attr('src', json[id]['img_front']).addClass('m');
                back.attr('src', json[id]['img_back']).addClass('m');


                if ($('.js-itm-hover').length > 5) {

                    if (ind === 5) {

                        filters_ajax();

                        <?php if (isMobile) { ?>
                            filters_mobile();
                        <?php } ?>

                    }

                } else {

                    if (ind === 0) {

                        filters_ajax();

                        <?php if (isMobile) { ?>
                            filters_mobile();
                        <?php } ?>

                    }

                }

            }

        });

    }, ind * 300);


});

<?php if (isMobile) {}

Answer is the line above, - it contains isMobile, which obviously had to be a constant, if not a variable, - turned out missed $ before isMobile, just like any non-constant variable in PHP, - be careful to syntax.