Toggle airplane mode. This will cause it to look for a(n) LTE signal.
If you know your MSL (call Sprint and ask for it, they will usually give it to you)
From your dialpad dial ##DATA#
Select Edit
Enter your MSL and click OK
Select EVDO
Select Preferred mode
Choose LTE only, or CDMA/LTE
Good luck!
I highly recommend switching back to the default setting when you are done "exploring" the LTE area.