This is likely due to corrupt software or possibly a bad battery.
This is not user repairable, so it will need to be returned to Garmin.
How did you test that the battery was fully charged? Over time, the battery can fail and will no longer store a sufficient charge, and this will prevent the unit from turning on. Around 4 years is a good lifetime for the battery and it may need replacing. The battery will need to be tested using Garmin's diagnostic equipment to ensure it is not failing. Once the battery is confirmed as good, they will check the software and reinstall it if it is corrupt.
Ok, glad you were able to at least rule out the batteries as a cause so it isn't a wasted trip.
Perhaps there is a courier service that could take the device to Garmin for you?
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