IMEI reg or extension? iPhone 15 PM

I have an iPhone 15 PM and I plan to stay in Indonesia for 5 months, I previously have got 3 month telkomsel esims but I am wondering if anyone knows the kind of price I should be expecting for that iPhone to register it at the airport and also if it would be better to just get another 3 month, then take it to telkomsel shop when that expires and extend it (I read that they give you one extension?) Any help appreciated thanks