I use my own system, based on LineageOS [1], which comes with settings to block internet permission on any application. It's one of LineageOS' features. ( On top of that I root my devices and use AFWall+ [2] which also creates a configurable firewall with iptables. )

[1] https://lineageos.org/

[2] https://f-droid.org/en/packages/dev.ukanth.ufirewall/