4G LTE cannot work on iPhone in Timor Leste

Where did the phone come from? Not all iPhones support all LTE bands. Check the model number against this list: http://www.apple.com/iphone/LTE/

Note also that a carrier data update may be required to support LTE, and your carriers may not have release one for iphones yet.

ahsan1910 wrote:

All carriers here are using band of 1800MHz. Does Apple need to have request from carriers here for unlocking the LTE services on iphone?

The carrier may have to install a carrier settings update on the phone to enable LTE. Apple's carrier support department should be able to give the carrier instructions on how to do this if they don't know how, but most carriers should already know.

Other than the one carrier in Indonesia, Smartfren, I do not see an Apple supported carrier for your location. That could be the reason that you are seeing this error. As Lawrence said, it could have to do with a carrier file, which is what tells the iPhone what settings to use. And not knowing what LTE bands the carriers use, that can also lead to users having the wrong version of the iPhone to use. iPhones are regional, and as stated, do not support all LTE bands. Your carriers may need to make contact with Apple to see about becoming a supported carrier.