Who sells unlocked one x's and could I use it on monthly 4g?
Yes, factory unlocked One Xs support 4G with no problems. I am not sure where you can buy them, though, as I am from Egypt. I'd say check Amazon and eBay, though, I am sure you'll find some for sale, both brand new and used.
Tegra 3 One X's don't support 4G it's worth baring that in mind, our 4G version of it is called One XL I'm not sure if that is the same in the states.
Yep, I am sure AT&T sells the One XL as well. I thought internationally unlocked ones supported 4G but apparently not. Sorry about my earlier post, I was mistaken!
international tegra 3 models don't support 4G.
the Snapdragon S4 model does support 4G, and it called the One XL in most of the world, or just the one X if you buy from AT&T (because they are crazy and just do what they want to cause chaos in the world). I wouldn't buy an AT&T branded model though, you can get an unlocked One XL with a HTC logo instead of an AT&T logo if you need the LTE support. make sure you check for compatible LTE bands though.
I would use t mobile network monthly 4g,..which is hspa+42
ah okay, so not actually 4G... actually HSPA+ (actually 3G, despite what all the carriers say).
I'm not sure if any of the models support HSPA+42... IIRC they definitely support 21, but but not sure about 42.
EDIT: This thread should help you: xda
This seems to be your answer:
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