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.