i dont remember exactly where it is on GB,but there should be a setting in wireless/networks to change from "cdma" to "cdma/lte"
-make sure youre set to cdma/lte.
-make sure the rear cover is fimrly attached
-try a reboot of the phone
-try toggling mobile data on/off
-try toggling airplane mode on/off
*give it a little time. ive noticed on my thunderbolt that a factory reset,or fresh RUU install will sometimes have flakey data for a bit
also dont discount the notion that theres trouble in your area,cold be a very random,yet unconnected coincidence... just a couple days ago i had no 4g all day long
turned out there was an outage in the indianna,ky,southern oh area