… which also means that reproducing a problem that only happens in the US (for example) in one specific location - or getting to a conclusion that there is a problem with the specific modem - could take time.
It would be the same challenge in my country, but worse. No staff of any type located here - so no possibility of any staff directly reproducing the problem. No familiarity with the three networks that are available here - and the possible quirks of those networks.
I think there is work needed to at least give more logging information about what is happening on the host side when the modem is not able to be persuaded to do its thing.
After an appropriate amount of time, and assuming still no response, you can also try pinging @JCS to see whether he can follow up on your behalf.
As an aside, last time I tried to use that, which was recently, - I wanted 4G-only now that the 3G network is defunct, and the 2G network was shut down a decade ago - it actually gave an error. I don’t know whether that’s because it (indirectly) depends on what is negotiated with the carrier or because something else went wrong. (So I left it at “2G, 3G, 4G (Preferred)”, which does work for me anyway.)