Javascript Display block when a button is clicked doesn't work on mobile

Question

I have this function that checks whether something inside a certain div is clicked. A part of the function checks if the targeted button has a certain class. This works perfectly, but when I go to mobile it doesn't work like I want it to.

btn.addEventListener("click", function (event) {
        if (!tgt.className.indexOf('nextbtn')) {
            navi.NextBtn();
            let dots = document.getElementsByClassName('owl-dots')[0];
            dots.style.display = 'block';
        }
})

I did a ! in the if statement because somehow it is reversed (I don't know why). If I remove it it won't do anything. On my computer it works perfectly, but when I go to mobile it doesn't change the style to block, but it does run the NextBtn function. Does anybody know how to fix it?