Worldwide LTE. - please help me to understand whether I've made a costly error

Question

I really fought hard to get an XperiaZ LTE in the UK. Curry's/PC Wotld, the nation's leading tablet outlet, were very slow to stock It. Though you could get the Wi-Fi only version, I knew I'd be travelling a lot and wouldn't always be abke to count on Wi-Fi so I waited, ignoring pleas from friends to go for an iPad. When I was finally able to buy one on September 20 it had to be dispatched from Curry's warehouses since no stores in the south held it. Maybe that's why I was wrongly informed that it would work anywhere, as long as I purchased a local SIM card. Well, I was sent to the US for 10 days and guess what? Three hours at an AT&amp;T store got me nowhere. At first the AT&amp;T card would not load. Then the switched-on assistant tried it with an already registered SIM and it worked. We couldnt beloieve it. Wi-Fi was switched off but I was online. So she got a new SIM and registered using an iPad in my name. Unfortunately this time, we found ourselves back to square one. The tablet read the SIM card, but would not hook on to the internet. Of course, because I'd come to the US armed with my new Xperia, I'd decided to leave my Vaio at home. So I was stuck, essentially left with a Wi-Fi only device after fighting so hard to get LTE. Can someone please explain this to me? Why wouldn't the AT&amp;T SIM load when, clearly, it is technically compatible as demonstrated to me by AT&amp;T? Does this mean I will never be able to use the LTE function other than in the UK? Why don't iPads have this issue? Essentially, I am left feeling my loyalty to brand Sony may have left me making the wrong choice...<br><br><br><br><br><br><br>

Answer 1

http://dl-developer.sonymobile.com/documentation/whitepapers/Xperia_Tablet_Z_LTE_WP_4.pdf
SGP321
GSM GPRS/EDGE 850, 900, 1800, 1900 MHz
UMTS HSPA+ 850 (Band V), 900 (Band VIII), 2100 (Band I) MHz
LTE Band I, Band III, Band V, Band VII, Band VIII and Band XX
AT&T's frequencies
USA Carrier Voice Frequencies 3G 4G 3G Technology 4G Technology
AT&T
850 MHz, 1900 MHz
850 MHz, 1900 MHz
700 MHz, 1700/2100 MHz
GSM/HSPA+
LTE
MetroPCS
1700/2100 MHz, 1900 MHz
1700/2100 MHz, 1900 MHz
1700/2100 MHz
CDMA/EVDO
LTE
Sprint
800 MHz, 1900 MHz
1900 MHz
800 MHz, 1900 MHz, 2500 MHz
CDMA/EVDO
WiMAX (2010–present)
LTE (2012–present)
T-Mobile USA
1700/2100 MHz, 1900 MHz
1700/2100 MHz, 1900 MHz
1700/2100 MHz
GSM/HSPA+
LTE (2013)
Verizon
850 MHz, 1900 MHz
850 MHz, 1900 MHz
700 MHz, 1700/2100 MHz (Planned)
CDMA/EVDO
LTE [3][4]
U.S. Cellular
800 MHz, 1900 MHz
Also AT&T has another issue, for LTE to work they have to register an AT&T branded 4G product for LTE to be available / enabled. 
AT&T's APN settings (this work on phones but I don't know about a tablet)
settings > more > mobile networks > apn > menu > add New > enter this
Name: T-Mobile
APN: epc.tmobile.com or fast.tmobile.com (for LTE devices)
Proxy:
Port:
Username:
Password:
Server:
MMSC: http://mms.msg.eng.t-mobile.com/mms/wapenc
MMS proxy:
MMS port:
MMS protocol: WAP 2.0
MCC: 310
MNC: 260
Authentication type:
menu > save
restart your phone
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