Frequencies and Network Questions

I'm planning on getting a phone overseas and I have absolutely no idea how the bands/frequencies work.

About T-Mobile coverage

So I looked at ^ and it keeps repeating 1700/2100mhz.

And on Ebay it says Tmobile is GMS 1900. http://www.ebay.com/gds/GSM-and-CDMA-Guide-/10000000009189079/g.html

So my questions are what key terms am I suppose to be looking for in the specifications sections.

GMS, WCDMA, HSPDA, CDMA

Do I need both 1700 and 2100(or whatever numbers it is) in order to call and use 4gdata/internet?

I have looked at other questions asked by other members about this and they said # s 850/1800 works.

One of the phones I am looking at is

Buy Xiaomi Mi4 - 5 inch FHD Screen 3GB RAM 13MP Camera Android Phone

^ This link shows the bands more simple

Xiaomi Mi 4 - Full phone specifications

^ And this one seems to have more info on the frequencies but I don't get it.

I live in NYC if that helps in anyway.

Thanks for clarifying for me!!