I am going to be in Koh Samui on 1-17. It’s an island off the coast of Thailand and I was there last week also. I got good service and speeds on AIS and they seem to be a major Thai carrier since my T-Mobile roaming service always lists them and connects when selected. I ran a Speedtest with 127Mbps down, 40Mb up last visit. They also have an English website for tourists that is pretty well laid out and has decent pricing and top up instructions.

AIS Link: https://www.ais.th/en/consumers/pack...l/tourist-plan.

Disclaimer: I have not purchased directly from them, but they seem pretty legit from what I can tell. My converter shows 100 Thai Baht = $2.85.