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No phones have band 66 support yet. I haven't heard of any carrier even deploying it yet.
 
I think I read that AT&T is rolling it out in the beginning of 2017.
Apple has always been behind in the mobile network game. Last with 3G, last with LTE, and they likely don't support the 4x4 MIMO that T-Mobile already has deployed. Hopefully the next iPhone will have it.
 
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