Shop not working on phones?

[arjena]

Just installed a fresh Prestashop 1.7.8.8. (The theme I want to use does not work with 8 so I stuck with 1.7).

Did all the work, like changing the theme, adding pictures etc. only to find it does not work on my phone.

So I installed another fresh copy.

Before changing anything, on a computer everything works as intended. But on a phone (tested with iPhone and Android) the slider that is supposed to be the topmost item is not there. My theme has a categories section under the slider (slider is a different module from the standard theme). After installing this theme, both the slider and this sections are not there on a phone. 

I changed back to the classic theme, slider is back on the phone. Back to my theme, slider is still there. But it disappears with no apparent reason the next time I load the page. All caching is off and apart from the sample data that comes with my theme it is still a virgin install. 

What could be wrong? 

https://www.werpdesign.nl/testshop