I wanted to do an actual write up of our experience with SIM cards in several SE Asian countries in Nov/Dec 2017. I used this thread to try and understand the lay of the land before we left, so wanted to contribute back. OP and other participants have long since taken their trips by now, but maybe this will help others.

Our itinerary was SIN-->REP-->LPQ-->BKK-->HKG-->SIN, over a total of about 15 days.

(Getting from the US to Asia, we flew JFK-->DOH-->SIN and back on QR, but that isn't relevant to cellphone issues or SIM cards.)

When we landed in SIN, we just got right in the taxi line and didn't get SIMs at the airport.

The next morning, we walked to a 7-11 near our hotel and purchased a Starhub card. We did NOT buy the "Tourist" card that was on offers pasted on the doors and windows. Just a standard prepaid SIM, for $15. It's valid for 180 days, but the validity period wasn't relevant to us because we'd only be in all of Asia for a couple weeks.

When we bought it, we gave the 7-11 lady our passports and phones and she activated the new cards. It took forever; she said her system was slow. But it was nice that there was nobody else in the place and we could buy a coffee to sip while we waited.

Upon activation, you get 15GB of free Bonus Data, which is local to Singapore only. (Invisible mentions this in his posts.) You don't need to do anything to take advantage of this, you'll just start drawing from it immediately when you turn the phone back on. This was more than enough data for us being in SIN for 2 nights/2.5 days, since we were on the hotel wifi a bunch.

At this stage, you can download the Happy Roaming app, and login with your new Singapore SIM phone number. But you don't need to do anything with it. The app will show that you have $17.65 of credit.

Our next two stops were Siem Reap, Cambodia, and Luang Prabang, Laos. According to the info that comes with the SIM and the Starhub website, there is no roaming service offered in these two countries. So, you'll need to buy a different SIM in both places.

In REP, once outside the arrivals hall in the taxi/pickup area, there is a kiosk selling SIMs. We bought a SMART brand. The guy gave us a menu with a number of choices on it, but we had seen somewhere a cheaper option than any on his paper, so we asked. He pulled out a different paper and we ended up buying a $3 data-only card. If memory serves, it was 3GB of data, and there was some validity period, but again, our time in country was less than whatever that was, so we can't remember. He installed and activated it, and it worked great.

In LPQ, there are several SIM-selling desks/kiosks in the arrivals hall. One was right by the baggage claim, and a couple of others were closer to the exit door.

We bought a $5 card from the Unitel guy by the baggage carousel, which I think was 3GB of data and valid for 7 days. I'm again hazy on the GB/days recollection. The price was definitely $5, and it was perfectly well sufficient for our needs over four days in Luang Prabang. Again, the kiosk guy installed and activated it for us.

The LTE speed in Laos didn't seem as fast as our Verizon LTE at home in the US, but it was perfectly sufficient and actually faster than our hotel's anemic wifi.

Before boarding our next flight, from Luang Prabang to Bangkok, we opened the Happy Roaming app, went to the Roaming tab and turned it on, and purchased a data package.

(Again, the app showed that we still had $17.65 worth of credit, since we hadn't previously bought any data package due to using the free Bonus Data we got with activation.)

We bought the 2.4GB/30 day package, since the card would expire long before we would ever use it again on any subsequent trip, so we might as well blow all the credits now. There were smaller/shorter packages available in case your situation involved going to a non-roaming coverage country next, and then returning into another roaming coverage country and needing to have credit left to buy another package.

On the LPQ-->BKK flight, we swapped out our Unitel SIM cards from Laos for our Starhub SIM cards from Singapore. And, we went back into the Roaming tab of the Happy Roaming app and followed the instructions to install a new profile in Settings. (NOTE: We both have iPhones (6s and 8), so I don't know how Samsung/Android might be different.)

By the time the time our plane pulled up to the gate in BKK, we were back connected.

While in BKK, we mostly only had a 3G connection, not 4G or LTE, but it seemed faster than what 3G would be back in the US. No complaints.

When we landed in Hong Kong, the SIM automatically connected to its HK partner, "Three" ("3") and we had 4G service for the days we were there.

On the ferry between Hong Kong and Macau, the service dropped down to a laggy 3G and the dropped completely for a short period in the middle of the crossing, but it reconnected soon with the Macau cell partner, CTM, and we had 4G all day.

And our last stop was a repositioning flight back to Singapore to catch our QR from SIN-->DOH-->JFK. The Singapore SIM worked fine again for our layover hours in town.

Anyway, that was our experience with SIM cards in several different Asian countries over about two weeks. Hope it helps future travelers.

No, the $15 15GB is valid for 30 days