That's a "Verizon" iPhone 6. You can get a Visible SIM card (runs on Verizon's network) and pay $25 for the first month, then just let it expire. The other carrier whose network has lots of prepaid options is T-Mobile, but without band 12 support you're going to have a bad time.
You're probably better off putting an American SIM (or eSIM, if you can) in whatever phone you use in Finland. Recent iPhones and higher-end Android phones tend to support enough LTE bands to work fine on either side of the Atlantic.