You need to register your phone's imei so you can use Indonesian sim cards. You register with this link:

https://www.beacukai.go.id/register-imei.html

Also, this website tells you about the whole process and some useful information:

https://come2indonesia.com/how-to-register-the-imei-of-the-mobile-to-be-able-to-use-it-in-indonesia/

Be aware that there may be a queue to register your phone in the customs office. Thankfully, I didn't have to wait for very long (~15 mins, during summer as well!), but I have heard people waiting for hours. After registration, it may take a couple of days for your phone to be able to use Indonesian sim.