It is well possible that your old French operator had no roaming agreement with the network in Boston that you are currently using (AT&T or T-Mobile, probably).
Issues like that can happen everywhere. I have travelled in remote areas in North England, where it often happens that Vodafone is the only network with coverage; the other networks covered remote places far worse. But my operator had no roaming contract with them so I couldn’t use the network.
This could have been changed though, it happened before I had a Fairphone.