It seems to be a fairly common complaint that once a battery dies, many no longer communicate with the app after replacing the battery, even if they power up and the LED works.
I just had a contact sensor stop working that had been going strong since initial early release. Except for battery changes. Anyway, the battery died, but this time when I replaced the battery, it still showed offline even though the led light was working, so I know it wasn’t dead. Couldn’t delete and add it back either, it would just time out. Finally gave up and replaced it with a new spare I had, but initially had problems with the new one to. It paired fine and showed up in the app, but was showing continuously offline in the app. After the 3rd time deleting and re-adding it, it’s finally working and showing online.
I know the bridge was working because other sensors were giving notifications.
I still have another contact sensor outside on my mailbox that was from original early release and that one is still going strong to. the batteries would sometimes fail if it got below zero degrees in winter, but always came back after a new battery. On that one, I’ve now wired in a two AA battery back instead of the button battery so it should last a ridiculously long time now, including cold weather.