I have both my cotact sensors down and not connecting anymore after replacing batteries. Did not get a warning or notification for low batteries at all… was checking for it ever so often after I read that it needs to be replaced before it dies but it did finally shut down with no warning at all. How fair is that? Now it won’t connect at all… the sensors do blink but won’t connect 
. I’m.using a Samsung Note 20 Ultra and tried it with another Samsung A70 as well…
The app should have notify you when the battery needs replacement.
Also the sensor should blink 3 times every 10 seconds indicating it needs a battery change.
How many times does the sensor blink when you insert the battery ?
It should only blink once … and auto connect.
If it blinks 5 times …sorry, but it’s bricked then I would open a ticket with Wyze replacement.