Here is the default apn for at&t on the ion. Be interested to see if lte still works for you.

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Current Phones/Tablets:
Xperia Z2 // OnePlus One // Xperia Z Ultra // Lumia 1020 // Surface Pro
Sony SmartBand // Sony Smart Wireless Headset pro MW1 // Smart Bluetooth Handset SBH52 // Z Ultra Power Cover CP12
Past Phones/Tablets:
iPhone 1st Gen/3GS/4S // Nexus One // Dell Streak // Atrix 4g // Lumia 900, 920 // Xperia Play, S, P, ion, TL, Z, Z1 // Xperia Tablet Z // iPad 1, 3

Click here for my Xperia firmware downloads.
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