What happens to a device when the manufacturer stops supporting it?
Answer first
It works exactly as it did before, but faults found in it are no longer fixed. Some devices stop working altogether if the service behind them shuts down.
A device may continue working normally after support ends. The manufacturer may announce the change on a support page or in the app, but the device itself may show no notice.
The difference appears when a new security problem is found: the manufacturer no longer promises a fix. Unsupported routers have been used in botnets and as criminal proxies.
The second consequence is for devices that lean on a cloud service. If the manufacturer shuts the service down, the device can lose features or stop working.
You can limit the damage without buying anything. A guest network, remote access switched off and closing any open ports are all free.