Sorry about the trouble Dean.
I have been using the app on my iPhone 5 with AT&T 4G LTE without a problem. Lets see if we can get this figured out for you.
Are you using auto-discovery or a manual connection?
If you go to Settings > General > Cellular and disable LTE it should revert to 3G. Can you connect with 3G?
When you say you can connect fine with wifi, is this wifi network the same one the computer is on? Can you connect using a different wifi network?
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