Try going to settings, backup and reset, reset network settings. Make sure LTE is enabled as an option to connect to first actually by going into mobile networks, network mode and make sure the option selected includes LTE.
Try dialing *#2263# from the phones' dialpad. Select  Band Selection. You're looking for the combination of LTE/WCDMA GSM Auto. If you see this combination, tap to select it and then tap again to confirm the selection. To exit, tap MORE > END. Do not reboot your phone.