Thailand is a big problem with tmo - landed at the airport and could not get a connection no matter which carrier I chose (so couldn't use grab.... grrr) Also on my return connection at airport was problematic - strange since their partners are selling local SIMS there which seem to work well- my guess is that tmo roaming doesn't get the best connection/service.
Stayed a month in central Bangkok and had HIGHLY variable service even in the same location with choosing the same carrier. Often had to change carrier to get a decent speed to use google maps (one worked one day another the next in the same location). Since I was there a month in a condo had a lot of opportunity to try same locations (btw tmo tech support was no help and promised call backs never happened )