Change home page on mobile

Site URL: https://f22-photography.co.uk/

I used to have a piece of custom-css which allowed my website to chang my landing page on mobile devices from the one on PC/laptops.

Today my website stopped working on mobiles and squarespace online chat told me the problem was my custom-css even though this custom code had been in years.

Does anyone know how some more up to date custom code i can use to help me change my landing page on mobiles ?

This was the code i used to use which worked perfectly up to now :(

<script>
if (screen.width<760) {window.location="/portfolio";}
</script>