If you read back a few pages you will see a couple of us have this issue. It was stated that it is most likely eCFSB ISSUE which you can read more about on the wall. Or, in this thread if you go back. I too lose signal very frequently when on the global and LTE settings in mobile network. If I switch to 3g the problem disappears.
This only happens in certain areas of my city, but both of the places I am most of the time (work and home), but not at school. This leads my to believe that the towers near me haven't been updated to properly support trip band phones yet. A few have 3g only upgrades.
If you have sponsorship look at the coverage nap to see towers in your area.
What you can do until your towers are upgraded, is set your mobile network to 3g. This will insure you still get your texts and calls on time. If you can't deal with 3g only, then I suggest you go back to using your single band phone until sprint gets around to fixing the issue.
Again, you can get more info on the issue by reading the article on the wall, or going through this thread, or the nexus 5 issues thread.
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