Hi @Donadoni, sorry to hear about this. 
Have you ever had an LTE connection with your Xperia Z3, or is this something that just started to occur?
To start with, download the Internet settings in your device and you can do that under Settings > More > Internet settings, tap "Accept" and wait for the download and installation to complete > restart your device.
You can also make sure that you are in an area with LTE coverage and that your SIM card/subscription is capable of LTE.
Let me know if that helps!
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