If you have a free package on a 4G or LTE network and your 3G network is paid. You can use this method.
As for if the signal coverage in your area is weak. so the network mode often switches to 3G or 2G. then we can lock LTE only permanently.

The method is very easy :
1. Use SIM 1 as the default connection setting

2. Download the Force 4G LTE only 2020 application via the Google Play store.
3. Run the application, press the SIM 1 only or Android testing button. You will enter two LTE Switcher options and also 2 Android testing options.
4. You can try all of them which support your phone.
If the telecommunication operator you are using does not support VoLTE (Voice over LTE) and uses LTE Only Mode, so regular calls will be blocked.