- AFWall+ firewall to allow list apps to internet using your preferred method (e.g. VPN, wifi, data, etc)
- PcapDroid to help monitor and analyze packets, or to just confirm things aren't communicating unexpectedly
- AdAway if you're not using your own dedicated dns over a permanent VPN connection
If not all 3 of these, AFWall is probably the best to go with. Having a way to not only block Apps, but also define your own custom firewall rules is very powerful. For example, I redirect all DNS requests to my own DNS with a custom rule (for apps, like Termux, using hardcoded DNS lookups instead of what the phone is set to)