@ Rommel Diw,
You do forget that the 640XL has been sold in two variants. With and Without 4G(LTE)
And there are a lot of countries where the 640XL was only available without 4G capable hardware.
So it is up to Abhishek to tell us which version he uses.
I really beleive he has got a non 4G (LTE) version so he never will get 4G capabilities with or without windows 10 mobile on his phone.
@Abhishek can you tell us the RM code (model code) that you can find on your phone
640 XL dual-SIM (no LTE) is model number: RM-1067
640 XL dual-SIM (with 4G/LTE) model numbers: RM-1065 , RM-1096 (both versions verified to work with USA LTE bands)