Hi Matthew,
I checked the front end of your website and you do not use our standard Portfolio setup. I see that you added the portfolio filter as a custom HTML inside the header builder and the Content Area element.
We do not support such a setup as it is not part of the original theme. Please kindly remove the setup from header builder and instead go to the page that you added and change the Page Template to Portfolio - Layout from Page Attribute options:
As soon as you change the Page Template the filtering options will be available for you. You can learn more on how to setup the portfolios here:
The final result should be like our demo portfolio page which the portfolio filter sits at the top and it whos correctly on mobile view:
https://demo.theme.co/integrity-business/portfolio/
kindly get back to us with the result of the steps above and URL/User/Pass of your WordPress dashboard using the Secure Note functionality of the post to follow up the case if you still have problems.
Thank you.