Can I use a iPhone bought in Thailand in Hong Kong?

Read this article about bandwidth compatibility. iPhone - Supported 5G and LTE networks - Apple

Warranty restriction! - Legal - iOS Warranty Document Us - Apple - "IMPORTANT RESTRICTION FOR iPHONE, iPAD AND APPLE TV SERVICE. Apple may restrict warranty service for iPhone, iPad and Apple TV to the country where Apple or its Authorized Distributors originally sold the device."

The warranty does not outright deny warranty coverage in other countries but there are many limitations to the effect Apple very likely will refuse to do repairs under warranty or even repairs at all except in the country of purchase. The device would have to be sent to a friend in the country of purchase to handle any repair claims because Apple will not handle international shipping. You may find if you take the iPhone to an Apple Store and speak with a manager that some repairs may be possible, but this is not a certainty.

mo-ony Said:

"Buying phone in foreign countries: [...]I was wondering if I buy international apple care, would my phone still be able to be fixed in Hong Kong despite the different parts, would they try to purchase new parts or am I just completely on my own? Additionally, I would also like to ask if the phone would work at all in Hong Kong as the bandwidth is different."

-------

Getting iPhone Out-of-Country:

It may or may not work. If you purchase AppleCare, then you'd lose it, as it cannot be transferred when you move to a different country/region. So, I recommend just waiting it out. The frequency would be no different. As for purchasing it, iPhones purchased in China all of a tap notification that wont go away.