Local radio links may remain
Wi-Fi LAN, Thread or bridged local devices can still communicate when the Internet is down if the relevant controllers remain available.
Cloud-only functions stop
Remote voice processing, cloud automations and external notifications may be unavailable.
Power and Internet are different failures
An Internet outage with working local power is not the same as a power outage.
Test the important functions
Know whether lights, locks, thermostats or leak alerts have usable manual or local fallback.
Keep in mind. Compatibility is not permanent. Confirm the current manufacturer, platform and certification information before purchasing, resetting or migrating a device.