An eSIM is like a regular SIM card but without the plastic. It's a small chip inside your phone that helps it connect to your mobile network.
iPhone XR
iPhone XS
iPhone XS Max
iPhone 11
iPhone 11 Pro
iPhone 11 Pro Max
iPhone SE (Gen 2)
iPhone 12 mini
iPhone 12
iPhone 12 Pro
iPhone 12 Pro Max
iPhone 13 mini
iPhone 13
iPhone 13 Pro
iPhone 13 Pro Max
iPhone 14
iPhone 14 Plus
iPhone 14 Pro
iPhone 14 Pro Max
iPhone 15
iPhone 15 Plus
iPhone 15 Pro
iPhone 15 Pro Max
Go to Settings > About > Carrier Lock
The top carries are AIS, Tre, DTAC
1. Make sure that there is at least one bar
2. Go to Settings > Cellular > make sure Data Roaming is turned on.
iPhone (upto 20), it is recommended that you should install 3-4 eSIM on each device. There will only ever be 1 active eSIM profile, though.