I have an IPhone 11 with AT&T that I want to use while in Chiangmai in September and October.  In order to get a local SIM card to work in it, does the IPhone have to be unlocked?  If it does, how hard is it to find someone in Chiangmai to unlock it?  In case I can’t get it unlocked, I’ll bring my older Samsung Galaxy S7 which is definitely unlocked to use.  

A locked phone won't work with any sim that isn't from that network (AT&T for your phone).

You can get it unlocked in Bangkok at MBK, but not sure where in Chiang Mai.

If you've been a "good" AT&T customer, you can call them and ask for their assistance in unlocking it for use in Thailand.  I have done that in the past.

I’ve got 5 hours between flights from Singapore to BKK and BKK to CNX on a Saturday morning, early afternoon.   I might go to MBK. If not, I’ll use my Samsung Galaxy S7 phone that is unlocked.  

AT&T will probably unlock the phone, so long as you have paid it off.

 

They even have a webpage explaining the process

 

https://www.att.com/deviceunlock/

 

There are also plenty of websites and apps where you can unlock for a fee.

 

https://www.imeiunlocksim.com

I called AT&T yesterday and, as I have a payoff amount of $488 on the IPhone, they won’t unlock it.  I’m not sure if I’ll have time to go to MBK.   I’ll try to find someplace in Chiangmai to get it unlocked. Failing that, I’ll bring my Samsung Galaxy S7 that is unlocked and use it.  I just need someone to replace the broken screen first. 

I brought my Samsumg galaxy 7S phone which works alright with a new sim card, even though the screen is shattered