no, you don't have to be rooted.
please follow this http://forum.xda-developers.com/show....php?t=2791761
the original fw (10a) that i had, did not have the LTE option anywhere. only 2G, 3G, 2G/3G in the network, but on 10b, it has the LTE option.
keep in mind, my phone is D855
edit the NVRAM, and unlock the LTE bands.
Don't buy ****ty versions with only 2GB of RAM and 16GB of storage, those are worthless.
The value that worked with my D855 is the one thread autor used to unlock all bands:
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