I have gone AOSP on the device recently, tho too busy to do much tweak, suddenly found that LTE was not enabled and it is not available on settings…(only 3G and 2G on prefered network)

A bit of Googling helps.

Dial *#*#4636#*#* and select LTE, then turn off Radio, reenable Radio and it is back with LTE, sweet.

Link:
http://forum.xda-developers.com/showpost.php?p=53067786&postcount=2898