Oh... and what about mobile data roaming? Is it enabled?

I was thinking that... but you shouldn't have to select that if you are using a local SIM I don't think.

Yes for some reason on this phone data roaming does seem to need to be enabled which it was.

I couldn't find a way to manually select a carrier - it showed whatever sim I had (eg Ais or true) with no option to change - how were u able to select?

Yes it says that but I still can't find anyone who has successfully used the US model - not everything on the web is correct - but it does seem like it ought to be some setting/band thing since all carriers show a strong connection - the wifi problem I had in VN years ago also showed strong connection but no data transmission so this seems like that :(

I don't have a OnePlus, but in general most Android phones' settings are the same:

Settings > Connections/Networks > Mobile Networks > Network Operators > Search Now

can you post the APN settings that you see now ?

my data only AIS sim says under :

Name
AIS 3G internet

APN type
default,supl,agps,fota,dun

did you go to Termanal 21 to get it set-up
AIS is on the 6th floor I think , with the Movie theaters

I no longer have the sim card - after 3 hours of not being able to get it to function AIS refunded my money
IIRC AIS set the apn to internet and aisinternet. Setting did not specify 3g and I don't think they changed the apn type (I think it was blank - maybe that was the problem.
I went to AIS store at Big C (small one which then referred me to Central store- which is where I spent an hour more and they ultimately gave up). Had purchased top up at the one in term 21 - but hadn't used it right then - since it was a card I already used and I got a message that I had 1g I didn't try to use it until a few days later.
I later went to the AIS store at ICON to see if I could try one of their sims - they let me, it didn't work, but they seemed to have little/no interest in trying to get it to work.

What phone do u have?

that was on a Moto E5 play unlocked from Tmobile USA , but it was a one year worldwide data sim and i think I added some minutes/text to it for Thailand

Thanks, so it wasn't a sim purchased in Thailand from one of their carriers - AIS, true or dtac?

Yes , I got it at AIS in Terminal 21

Thanks, but the problem is the specific phone oneplus. I was confused by "but it was a one year worldwide data sim" I didn't know that AIS sold any worldwide data sims- do you mean sim2fly? How were u able to get a one year version I'm only seeing 7 or 15 day packages on their site (have u used it in the US?)

I too, had a phone (samsung s8) that worked with the AIS sim card so I did not anticipate any problem when I switched to the new phone - but it just won't work.

bal
Written in the purple column on the right SIM2FLY the best-value roaming SIM for everyone! (desktop)

Thank you so much for the link - when I had asked in an AIS store if they have any sim cards that I can use in Malaysia they said no :( I guess the sales staff don't know about all the products their company sells even though it says I can buy in any AIS store:confused:

As to my original problem - oneplus has been of no use (keep suggesting the same things that I told them don't work). I was able to borrow a friends AIS sim today so I could make sure all the apn settings were correct - and still no success :( The signal shows 5 bars volte also shows and no combo of turning data roaming on or off, etc. allows me to download data (altho the up and down arrows indicate that it should be working), nor to use any call function.

The only thing that makes sense at this point is that maybe the IMEI was not supplied to AIS (or any other Thai provider) if what another poster wrote is correct or ?????
When I get home I guess I have to go back to my old phone next time I'm in Thailand. I'll be in Malaysia soon so I may try again there to see if my phone works there,

Thats the one I got , and it does work in the USA on tmobile , I used it today !