The AT-2 is pretty old, so it may well just be broken, but have you checked to see if it has a manual setting? Many tuners have the option to set it to a specific pitch. I haven’t seen one like yours in so long I can’t remember if it does or not, but check to be sure that’s not it.
If you’re living in South Korea I would guess you have a smart phone. If it’s an iPhone or an Android phone you can get a tuner app for that so you don’t have to carry around a separate device. Cleartune is available on both platforms and probably the most popular.