Can you flash the D855 to be a D851?
The short answer is NO. Long story short: I had a great reply written for you but i can not post links so Android Central didnt let me post it for i do not have 10 posts.
I had a d855 that couldnt get LTE on tmobile. The fix is : search xda : Add all GSM and LTE bands to your phone.
That thread will help you get all bands.
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