Turn off BXSlider on smartphones

Question

I use the Eight Sec Wordpress Theme on the page https://kegel-und-bowlingbahnbau.de/. The BXSlider is integrated there.

However, the slider is not displayed correctly on the smartphone. The picture is very small and it looks like it was zoomed in.

That's why I would like to switch off the BXSlider on the smartphone and replace it with a single image.

I used the following code in the custom.js file:

jQuery(document).ready(function($){

 var isMobile = /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent);

 if (!isMobile) {
     $('.testimonial-slider').bxSlider({
         //adaptiveHeight: true,
         pagers: false,
         controls: false,
         mode: 'fade',
         car: true
     });
 }

Unfortunately, that doesn't do anything at all.

I also tried in the jquery.bxslider.js file the settings (at the beginning) of:

 // TOUCH
 touchEnabled: true,
 swipeThreshold: 50,
 oneToOneTouch: true,
 preventDefaultSwipeX: true,
 preventDefaultSwipeY: false,

to change

 // TOUCH
 touchEnabled: false,
 swipeThreshold: 50,
 oneToOneTouch: false,
 preventDefaultSwipeX: true,
 preventDefaultSwipeY: false,

Also without result.

What else could I do?