I had the same issue two days ago when I flashed to the latest MIUI Rom. Here's how I fixed it.
Go into settings, mobile networks, make sure subscription is set to SIM. After that open the phone, type *#*#4636#*#* make sure it's set to LTE/CDMA/AUTO
Reboot phone and all should be well.
Sent from my 16GB Blue Verizon Galaxy S3 running MIUI v5 with KT kernel
16GB Blue Samsung Galaxy S III (Verizon)... With a different ROM every couple days....
Custom Built PC- Running a tweaked, de-bloated Windows 8
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