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I have found the same with my Xperia Z2. If I set it on LTE and it finds 4G then it won't Fallback to 3G for Data...it just sits there with the Signal Strength (i am assuming it is 3G Signal) until I perform a Restart.
We are now utilising what Telstra call 4GX which is at 700Mhz....I was assuming this may be the issue as it may not be fully developed as yet even though Telstra say it should roam over to 4GX seamlessly.