Why does my phone say 3G instead of 4G

If you have a 5G capable phone or tablet with the appropriate SIM card, and you are in a 5G service area, you don’t have to do anything to take advantage of the increased speed and connectivity. Your device is preconfigured to take advantage of all the benefits of 5G automatically .

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

Switch between 3G/4G/5G – Samsung Android

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.