Notification if data not received

Hi All,
Im trying to get a notification from blynk if my temp sensor hasn’t uploaded new data to blynk after a certain time IE if the wifi connection is down for some reason.
The sensor isn’t connected all the time it wakes every 15 mins connects to blynk uploads the latest readings then goes back into deep sleep

Ive tried the data validation to change the temp field to 1234 if no data received for 20 mins and then tried to automate this to notify me if this happens but i cant get it to work.

If the sensor doesn’t upload new data for more than 15 mins for whatever reason is there a way blynk can let me know it hasn’t received the data and there may be a connection issue

any help much appreciated