Thailand is a place where a single trip can cross three very different kinds of terrain: a dense city, the northern mountains, and islands. Coverage is not the same in all three.
Bangkok
Coverage is excellent, including on the BTS, the MRT and inside the big malls. You will not have problems.
One note: at peak hours in busy stations like Siam or Asok, speeds can drop because thousands of people share one cell. That is not a fault in your plan — every network behaves the same way.
Chiang Mai and the north
The city itself is fine. Outside it is a different matter:
- The road up to Doi Suthep: coverage exists but drops on some bends.
- Pai and Mae Hong Son: the mountain roads have long stretches with no signal at all. If you are riding a scooter, download offline maps first.
- Hill villages: many have no coverage. Better to know that before you are standing there.
The islands
Phuket, Koh Samui and Krabi have good coverage in the tourist areas. But:
- Remote beaches and smaller islands (Koh Lipe, northern Koh Tao, the quiet bays): weak or no signal.
- On the boat out: losing signal mid-crossing is normal. Do not arrange to take a call while you are on the water.
How much data
Thailand is a place where people use less data than they expect, because hotel and café wifi is both common and fast.
- Ordinary sightseeing: 2 GB per day is more than enough.
- Shooting video and posting photos: 3 GB per day.
- Many days across several places: buy one longer plan rather than several short ones — it works out cheaper.
If your trip covers Bangkok and the islands
You do not need two plans. A Thailand eSIM works nationwide, islands included. It is only the places that have no coverage at all where no plan can help.