I had the Samsung s4 repaired due to freezing and not receiving calls etc. When I got the phone back , I noticed it is not running on the LT network anymore but on G4 - why would that be ? Is the LT not faster ?? The phone is only 3 month old and it had to be sent out twice!!!
Either a) there is no LTE signal to pickup/it's weaker than 3G/4G or b) you have it turned off in your settings. I suggest checking to see if you actually have LTE enabled on your phone. You settings could have been changed after repair. Which can be found under mobile network.