In most cases, your phone might fall to 3G or 2G at some point when the signal strength needed for the 4G service is not available in your area . That is, there is no reliable network coverage where you are at the moment.
Read moreWhy is my phone only showing 3G?
It’s happening because whenever the signal quality decreases, your phone searches for an Alternative, with a better quality . Then let it be a 3G connection. So, You need to go to settings>mobile network>and swich from Auto(LTE/3G/2G) to LTE.20 Eyl 2019
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