It’s been a few years since we visited other parts of Europe. We used to pick up an Orange SIM in the airport and use it throughout our trip. We use tMobile in the States. How does Wi-fi service work in Croatia 🇭🇷? Who is the main service provider?
You're confused about how stuff gets to your phone. You are thinking about cellular service. "Wifi" is does not require cellular service or a SIM - you can access wifi without either cellular service or a SIM (you just need to turn on wifi on your device, and be within range of a wifi router that's public/open or you will need a password, as at a place like Starbucks or your hotel).
For cellular service, there seem to be 3 main providers: T-Mobile, VIPnet and Tele2 (service from other European providers should also work).
Since you already have T-Mobile, you should check with them to see what your options are.
TMobile One is what you want to have activated (in the US) so your phone works on the CELLULAR networks in Europe. No need to change SIM or anything.
Welcome to Croatia. Korcula is an excellent pick. I suggest asking at home how much is roaming (call and surf) for visiting Croatia. It is much easier to activate that option at home before you go travel. Less stress later. If you need Wifi, you can turn Hotspot on your phone. Every phone has that option.
The second option is to buy a new SIM in Croatia. You can buy at the airport — no scams in Croatia. Two popular brands are T mobile and VIP. Their network covers the full territory of Croatia. When I travel, I usually buy 4 and more Gigs for one month. 8 Gigs are ideal. The texts you typically get unlimited. Calls are a tricky part. You should take option unlimited to all networks (inside Croatia). 098, 099 is T mobile and 091 is VIP.
Hope it helps.