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.