Edge 540 key stopped responding

The select key (top right) on my Garmin Edge 540 has suddenly stopped working, but everything else is fine.

It stopped responding overnight between two rides for no clear reason.

I’ve had the device for less than four months. The solution on Garmin's support page didn’t help:

https://support.garmin.com/en-US/?faq=xVV2WIeq5h9QbF9bHfcmH7

I contacted Garmin Support (Norway), but they won’t assist without a proof of purchase, which I don’t have since I bought it second-hand (unused by the previous owner). Garmin claims they need proof to confirm the device is less than 5 years old (the warranty limit in Norway).

When I told them the device was released in April 2023, they said it might have been purchased in another country with a different warranty policy.

You'd think Garmin could verify this through the serial number, but apparently not.

Repairing the device costs almost as much as buying a new one.

Any suggestions on what to do?