We want you to have 4G coverage every where you go. I also have a iPhone 6s Plus and I've also ran into this issue as well. Let's figure this out together. Does this mainly occur outside or indoors? Are you able to make calls, text, and use data?
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Is the LTE disabled on your phone? Settings > Cellular > Use LTE. Set to either "Data and Voice" or "Data" if it's set to "Off."
Go to Settings > General > Reset > Reset Network Settings to force the iPhone to re-activate. If this doesn't fix the problem, consider having Verizon re-activate your device. Call from another phone or stop by a store to do that.