Hello James. We found the solution.
We’re using a child theme on the site so we could customize the css a little bit. What we had also done was copy the footer.php file to the child theme directory and adjusted the branding link at the bottom to show different text.
It was this file that was causing the menu to not work properly. Once we removed this file and updated the original footer.php file instead, the mobile menu worked on the tablet and phone.
Thank you for your help.