My experience has been whenever you select an iPhone with a carrier you are connected to that carrier until released, and you cannot connect with any other carrier.
Never again will I get new carrier linked iPhone, and think that I’ll be able to use it anywhere with any server any provider of service, because it doesn’t work. There’s a process you must go through. In my case AT&T has to let Apple know, and then Apple does something to place the phone to an unlocked condition, but only after you have completed the contract.
I go out of country and absolutely, again in my experience, you do not want one of the international AT&T plans. I suspect other major carriers operate in a similar fashion. You can’t possibly use any of their plans, and not exceed one of the limitations, and then automatically get placed into international Rates, and end up with a bill of $500-$700 for a two week visit out of country. The only way around this is to have an unlocked iPhone directly from Apple, never from a carrier, and take the Sim card out in Miami before you go over the ocean. And you have to remember to forward calls to the AT&T number they have so you don’t get charged international rates for people calling you. If you do not do this, you will get charged for international calls whether or not you answer the call or not. You must do this before leaving Miami or whatever airport you’re at that leaves our airspace. on any of the Caribbean islands, you can get a local carrier for two weeks for about $20, Claro or Altice, with unlimited data and hundreds of minutes of calling, which you won’t need much of. You can do everything on one of the messaging apps that has calling and video chatting.
going on a trip sometime this year and see how the eSIM situation affects travel. I’m assuming you can turn off the eSIM to your local carrier here in the USA, and activate an available eSIM when you get to your destination. Again, it must be an unlocked iPhone from Apple directly. If not, then One can use an Internet unlimited data plan using one of the portable Wi-Fi hotspot devices. You can still get those that use SIM cards, then you just connect to it over its Wi-Fi. If you must make local phone calls, they buy a cheap burner type phone and buy a few hundred minutes for a few pesos.
I have not tried this any other way in the past five years or so, and what I have stated here may have changed, but I doubt significantly.
Best, Seth