Why is my LTE connection not working?

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.

Restart your device Simply press and hold the power button of your smartphone and then tap Restart. Wait a couple of seconds and turn it back on. Check your status icon, but also test the speed of your LTE connection by opening some websites or downloading some smaller apps.14 Tem 2019

If you select LTE/GSM/WCDMA (auto connect), your mobile phone automatically switches between the three network modes provided you’re within range of a 4G network . If you select GSM/WCDMA (auto connect), your mobile phone automatically switches between the two network modes provided you’re within range of a 3G network.

Moving from 4G to 3G while on hook means you’ve been detached from the 4G cell mainly due to to signal strength and/or signal quality . When you make a call, your phone detached from 4G to 3G because 4G, unless voice over LTE is activated in the network, can’t facilitate phone calls directly.