As others have mentioned, if you're getting signal, the phone's radios support the country's cellular network. Thailand uses LTE bands 1,3,5,8,40,28. Other than some of the Verizon-crippled phones, even the cheapest, nastiest LTE phone will support at least two of those frequencies.

I'm still leaning with the others -- configuration error. If the cell networks were going to deny the phone, it wouldn't be able to establish a 4G/LTE connection with the tower. Instead you'd get the No Signal or Emergency Calls Only messages.