Thanks but the AIS sim I had that worked in my old phone (samsung) doesn't work in the oneplus phone in Thailand I had no trouble with it in the US - also with a carrier that uses tmo which is why I was so surprised that it didn't work in Thailand :(
So ultimately I have to reiterate - do not buy oneplus phone in the US if you want to use it in Thailand!

Update - doesn't work in Malaysia either :(
 

So I spent a week also trying to trouble shoot with Fi while in Thailand - despite downloading a bug report and other issues they requested they were of no help whatsoever (except for taking up a LOT of my time).
Now I'm in Malaysia where I have exactly the same problem - using Fi phone shows strong signal but NO DATA CAN BE TRANSFERRED. Again Fi support is of no help. AFAICT there is no point in trying local sims if Fi won't work.
So my only conclusion is that THIS PHONE DESPITE BEING UNLOCKED WILL NOT WORK outside the US - at least in Thailand and Malaysia.
I have no idea why it connects with a strong signal but allow to transfer of data.
BUYER BEWARE.

Did you buy it online? I have been using OnePlus from the one and now 7 plus and it works in every country. Even in Thailand and Philippines and Maldives and every country I have been to.
i agree with you that the people at the mobile shops don't know much and especially the OnePlus.
You need to take it to MBK and ask a tech shop there. They will fix it as they sell OnePlus a lot as lots of foreigners buy them there too and are familiar with the OnePlus unlike the mobile shops like True, Dtac or AIS.

Change doesn't work in Thailand to doesn't work ANYWHERE!
 

So apparently I have a defective phone (which oneplus refuses to even acknowledge or do anything about)
It does NOT work with cellular data in ANY country - including the US where it was sold. I had tried everything except using a t-mo sim (which they claimed would work for sure), but today it is clear that that didn't work either. Went to t-mo store (which used to sell this same model) and nothing worked.
The weird thing is that it looks like it should work - strong bars, up and down arrows BUT no data is actually transmitted!!!!
So I guess I have to change my warning from Thailand to make sure you don't have a defective phone in the first 7 days.
I think when I first used it in the US I was always on wifi when using data and texts worked when I was out and about so it never occurred to me that data on cellular wouldn't work (I assumed that b/c texts worked that meant data was working).
It wasn't until I tried using an AIS sim (which had worked in my old phone) that I discovered that data wasn't working over cellular (tried 3 other thai providers, 2 Malaysian and 3 US). So whatever hardware problem exists I have it! So much for quality control or customer support :(
I bought the phone from oneplus website in the US (AIS sells them and didn't understand why their sim worked in their phone but when switched to mine it didn't work and when my sim was put into their phone it worked fine - clearly suggested there was a problem with my phone -all APN's set correctly, etc). They had never seen a problem with the phone using their sim.

That kind of poor customer service for a defective product purchased within a year isn't a resounding endorsement, either.
https://www.oneplus.com/legal/terms-of-sales

Perhaps some of their support forum team members could help nudge a ticket along https://forums.oneplus.com/threads/t...apter.1168120/ ?

It should be covered under their 12 months warranty even if you can't return it.