If the Passiv hub loses internet connectivity, your system will keep running with the schedule and temperatures already set in the system.
If your internet has gone down, check to see if other devices are connecting and if not check your router and contact your provider if this doesn’t work. If other devices are online but the hub is unreachable, check the TP-Links (if you’re connecting this way).
Don’t turn the hub off and on if you lose internet connectivity. This will cause the hub to lose your stored schedule (time + temperature etc) which stops it from demanding heat/hot water automatically.
If the hub loses connectivity you won't be able to use the app to make changes. You can still use the thermostat to increase/decrease the temperature in the home. Your hot water should continue to heat as per your schedule. Once your internet connection is up and running you should have full-feature support again.