If you can't prevent uninstall or install, here is a setup you can use:

My suggestion is to use the DNS settings on your iPhone, since those don't depend on an app and will keep your device connected to the content policy. That's under the Config Generator > iOS > Network Settings> Enforce DNS Settings (make sure you select them properly so it is your content policy)

Next thing you need to do is block new VPN apps from adding VPN configurations so that they can't override your DNS settings. Currently you need to do this using Tools > iPhone > Config Presets > download the config called "Restrict New VPNs".