@AppleSupport I have a Sprint 6s and I haven't been able to receive lte data since the ios 9.3 update
— I'm Broke Cuhhkz (@bUCCY500) March 25, 2016
Basically, a bunch of Sprint subscribers with an Apple iPhone realized that they could no longer take advantage of LTE connectivity on their handsets after performing the update, leaving them with “mere” 3G connectivity wherever they are. It would seem then, that this is specifically a carrier issue and will have nothing to do with Apple. Hence, the responsibility would like with Sprint to ensure that this particular issue is fixed.
Sprint did send out a free text message to its iPhone users, mentioning, “Your iPhone may be having data connection issues w/ the recent software update. We’re working quickly to fix. We apologize for the inconvenience.” Hopefully the fix comes sooner rather than later!