Phone is locked with a $700 balance. What's the best way to get data for a 4-month study abroad in Korea?

Hey everyone, I'm heading to Seoul for a study abroad semester (late August to just after Christmas) and I'm stuck on what to do about my phone. I use T-Mobile, but my phone is carrier-locked from a trade-in promo. There's still about $700 left on the balance, and from what I understand, paying it off early means I lose my monthly promo credits, so I’m not sure that makes sense. What do people usually do in this situation? I've heard of people just using T-Mobile roaming until they get cut off, or buying a cheap secondary phone or portable hotspot once they land. Has anyone navigated this recently? What's the cheapest and most reliable workaround for a 4-month stay? Thank you!