It was the same for me (the device had not been in use for several months). Conclusion: the battery (for the power failure option) was completely flat. Actually the death sentence for the device, because the battery is permanently installed or glued, a single cheek from the manufacturer. The man (an electrical tinkerer, thanks for that) removed the battery and damaged its casing in the process (no matter!), then brought it back to life and now the Winbot is working again.

Yes, this seems to be a deliberate approach by the manufacturer to sell new devices. The batteries fail shortly after the warranty expires

Nothing, it must remain connected to the power supply.