WPEngine hosted site not displaying on new Mac or new generation mobiles

We host 2 sites on WPEngine. We recently purchased a new Mac Notebook and found the sites would not open as they should. We see a jumping list of headings, navigation links etc. no images. We then discovered the same effect on new Ipads, mbiles etc or a message saying unable to connect to server. The sites display correctly on our older Mac even though fully up to date with operating system as well as older mobiles and tablets.

The strange thing is if we log onto the backend, Admin Panel, the site displays and loads correctly. We believe this is a problem with the server and the way WPEngine host sites but they deny responsibility and say it is the theme ( Real Estate 6) but as the sites display correctly when we are logged in I do not believe the theme is at fault nor does the developer as no other users have raised the question.

Just wondering if anyone else has found a similar problem or can suggest a cause or a remedy.

Thanks