Just returned to the US from a 4-week trip in LOS. My stateside cell provider is T-Mobile. My plan includes unlimited data/text in Thailand. Plus T-Mobile includes Wi-Fi calling for calling the States, which is free. Functions like you're stateside. Added a Thai eSim to my phone. Worked great.

My experience is from 2015. If you're going to Cambodia by air, you'll get a visa on arrival for Cambodia, no need to get a visa prior to arrival. Cambodia uses US Dollar. If you land at Phnom Penh or Siem Reap, there are no scams at the airport. But if you enter by land, I've been scammed at the crossing south of Trat.

If you return to Thailand by air, you get a new stamp for 30 days. If you're only going to be SEA for 16 days, I recommend you fly and not waste a day or two in a crowded van on Thai/Cambodia's roads.

The Thai's limit entry by land to 2 times a year. By air entry's are not limited.

Are you traveling on a US passport? Different rules for different countries.