Not OP, but as far as I know that's how it works, yeah.
When I was in China, using a Chinese SIM had half the internet inaccessible (because China). As I was flying out I swapped my SIM back to my North American one... and even within China I had fully unrestricted (though expensive) access to the entire internet.
I looked into it at the time (now that I had access to non-Chinese internet sites!) and forgot the technical details, but seems that this was how the mobile network works by design. Your provider is responsible for your traffic.
When I was in China, using a Chinese SIM had half the internet inaccessible (because China). As I was flying out I swapped my SIM back to my North American one... and even within China I had fully unrestricted (though expensive) access to the entire internet.
I looked into it at the time (now that I had access to non-Chinese internet sites!) and forgot the technical details, but seems that this was how the mobile network works by design. Your provider is responsible for your traffic.