Because I am not talking about WiFi at all. Wifi isn’t even in the picture. Wifi is 100% irrelevant to my original statement.

I am talking about the frequencies handed out to phone ISPs by each country’s telecommunications agency that determines how cell phones connect to cell phone towers in the first place.

Each country has a different mix of spectrums that they hand out, even if the same company operates on both sides of a shared border. So a phone with ISP X that works just fine in a person’s home country might be totally unable to find any kind of a signal to connect to on the other side of the border. No phone calls, no SMS, no mobile data, nothing.

This is because phones are manufactured to connect to a limited number of known domestic frequencies - so-called “international” phones are extremely expensive for this requirement of having all chips for all possible frequencies - so the phone that can work perfectly fine on one side of the border may be totally unable to connect to the frequencies that ISP X uses on the other side of the border - there is no overlap with the phone’s supplied hardware.

It pays to properly read the original comment.