Modal Gallery - next and previous button to this script?

Question

I have this code, I need to input next and previous buttons or link into modal image, when I want to change photo (not in preview but in modal). For me is not a problem to input them into HTML script, but the problem is I can't so much JS so I am not able to create a script that can move an image from one to another infinitely in loop when clicked next/previous button. I accept code written in the newest JQuery(mobile) cause I use it too on my page. Many thanks for the help. Here is the link as it works now https://jsfiddle.net/8ryutc5g/2/

<div class="slideshow-container">
<div class="mySlides fade">
    <div class="numbertext">1 / 5</div>
    <img class="ImgThumbnail" src="http://srovnej.jednoduse.cz/img/svarec3.jpg" style="height:300px; width:100%;">
    <div class="text">Dělník</div>
  </div>

  <div class="mySlides fade">
    <div class="numbertext">2 / 5</div>
    <img class="ImgThumbnail" src="http://srovnej.jednoduse.cz/img/svarec.jpg" style="height:300px; width:100%;">
    <div class="text">Montér</div>
  </div>

  <div class="mySlides fade">
    <div class="numbertext">3 / 5</div>
    <img class="ImgThumbnail" src="http://srovnej.jednoduse.cz/img/svarec2.jpg" style="height:300px; width:100%;">
    <div class="text">Uklízečka</div>
  </div>
  <div class="mySlides fade">
    <div class="numbertext">4 / 5</div>
    <img class="ImgThumbnail" src="http://srovnej.jednoduse.cz/img/zamecnik.jpg" style="height:300px; width:100%;">
    <div class="text">kuchař</div>
  </div>
  <div class="mySlides fade">
    <div class="numbertext">5 / 5</div>
    <img class="ImgThumbnail" src="http://srovnej.jednoduse.cz/img/zamecnik2.jpg" style="height:300px; width:100%;">
    <div class="text">Striptérka</div>
  </div>

</div>

<br>

<!-- The dots/circles -->
<div style="text-align:center">
  <span class="dot" onclick="currentSlide(1)"></span>
  <span class="dot" onclick="currentSlide(2)"></span>
  <span class="dot" onclick="currentSlide(3)"></span>
  <span class="dot" onclick="currentSlide(4)"></span>
  <span class="dot" onclick="currentSlide(5)"></span>
</div>  

<div class="modal">
<span class="close">×</span>
<img class="modalImage" id="img01" style="max-height:800px; max-width:100%">
</div>



    <script>
var slideIndex = 1;
showSlides(slideIndex);

// Next/previous controls
function plusSlides(n) {
  showSlides(slideIndex += n);
}

// Thumbnail image controls
function currentSlide(n) {
  showSlides(slideIndex = n);
}

function showSlides(n) {
  var i;
  var slides = document.getElementsByClassName("mySlides");
  var dots = document.getElementsByClassName("dot");
  if (n > slides.length) {slideIndex = 1}
  if (n < 1) {slideIndex = slides.length}
  for (i = 0; i < slides.length; i++) {
      slides[i].style.display = "none";
  }
  for (i = 0; i < dots.length; i++) {
      dots[i].className = dots[i].className.replace(" active", "");
  }
  slides[slideIndex-1].style.display = "block";
  dots[slideIndex-1].className += " active";
}


var modalEle = document.querySelector(".modal");
var modalImage = document.querySelector(".modalImage");
Array.from(document.querySelectorAll(".ImgThumbnail")).forEach(item => {
   item.addEventListener("click", event => {
      modalEle.style.display = "block";
      modalImage.src = event.target.src;
   });
});
document.querySelector(".close").addEventListener("click", () => {
   modalEle.style.display = "none";
});
</script>