I've got an old iphone SE 2 second hand, pretty sure the seller is dead at this point. I'd like to use a new sim card, but the carrier is locked (I think Tmobile)?

Glancing online, it seems like the retailer will want proof of purchase. Is there anyway to do this easily from home, or should I start looking at jailbreaking it? (This is for a friend; I'd rather keep this as simple as possible)

Follow up for any future wanderers: This worked, but only because the seller was indeed dead. They asked me to pay the balance on the account, took a credit card payment, and then unlocked it ~5 min later. Not too painful.