October 30, 2019 at 08:06 PM
Well I don't know of any app, but just make sure your device never connects to anything bellow 3G and LTE (i.e. 2G). I am not sure if there is any straight forward way of doing it, such as installing an app.
* There are some apps that do suggest that they lock you on 4G such as com.kliksob.forgswitch
* There seems to be some suggestions online using Xposed (probably frida as well) but that would require a rooted device and some skill (if you are so concerned about security then you should probably avoid rooting your device)
* The most reliable way is probably building a custom ROM for your device or finding one that has 2G disabled.
This should keep you safe but I'm no expert so you should probably test this somehow.