It’s technically impossible to do this reliably anymore due to recent Android changes outside our control.
We made firewall profiles to make up for it. You should make one profile for mobile and one for WiFi.
https://blog.glasswire.com/2020/03/06/glasswire-3-0-for-android-is-now-live/
“We also found that Google made it impossible for us to block data by WiFi only, or Mobile only with our firewall.”
Others may still have the feature or claim it’s possible but it very clearly leaks and it’s unreliable. It would be wrong to offer this feature when it leaks and doesn’t work due to Android changes we have no control over.