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This generally is owing to being on battery. The display dims down often enough or always, unless the device is getting plenty of power from a AC Adapter as power source.
Power-saving measures are enabled on Battery, so that setting or option might need to be disabled on the Garmin device.
It might need more charging before it is good for its first use. Rechargeable batteries from the store, I usually state as a principle, need to be on the charger for 6+ hours before first use, regardless of whether the charger shows them as charged before then.
(Note: you said, the Garmin's screen is real bright on an 'external power source' - which power source is that?)