WordPress Site Not loading correctly on data connections

So, I have a very strange error with my current wordpress site when viewing on my iPhone 6.

On wifi, everything seems to be loading correctly. However, when I try and access the site via a data connection it doesn’t seem to wait to load the full page to show content (i.e. it’s not too fussy over what scripts it loads). This leads to missing elements of the css, missing images, missing fonts. The site looks a complete mess. Sometimes it doesn’t load any styling and I get completely unstyled pages but all the content.

What’s really weird is that I checked it on my partners iPhone 5c, on 3g and 4g and it worked fine. She is on EE and I am on O2. It also loads fine on my iPad which is EE.

So it seems I am having issues with O2 data loading the page. Has anyone come across anything like this before?

Site is https://www.danielalcorndesign.co.uk