Ring doorbell going offline but it's actually connected

Recently two days ago  my hard-wired Ring Doorbell Pro constantly goes offline. However, from my router, I can see the Doorbell has very good connection. It is assigned an IP address, and I can ping it. Also, all my other Ring Stick-up Cameras, which connect to the same Wifi, are online. What's more mysterious is that the Doorbell can continue to detect motion and respond to the button. And once it has detected motion or being pressed, it goes back online again. But, it goes offline after a few minutes. Is there a bug in Ring's server side that determines whether a device is online or not? Or is there a bug in the latest firmware? I have tried resetting and everything else.