What I found was every now and then the LTE in my area takes a dump and I know it because I'm set to LTE with voice and have no signal unless I change it. When it dumps it may last a few minutes or a couple of days, usually somewhere in between. If LTE is new in your area you could be going through the testing growing pains like my area did for a couple of months. The only time it dumps now is when it goes down nationally or regionally.
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