The information you give is not clear enough to give a answer. Did you flash your ROM, what phone you have, do you not see the 4G in settings, do you not see the 4G in the top bar of your screen. What did you already do soft reset/hard reset. Do you have other problems?
So without proper information you get above answer which may or may not help.
Re: After 8.1.1. update 4G is no longer available only 2G/3G
Did you flash a new region ROM before doing the upgrade? Different regions use different LTE / 4G channels and you may have lost your 4G because of that.