I'm actually running with a US Mobile SIM right now (which is T-Mobile). It "works," but I don't know if it works everywhere it should (it was dead on part of the 101 between Petaluma and SF over the weekend). T-Mobile does have overlap of frequencies, and their main band is the same as AT&T, but it isn't identical. Here's a good site
with what carriers use what bands, as well as Wikipedia
I'm guessing JLR disabled the bands they don't need for AT&T and whatever provider they use in Canada. I'm guessing I'm not lucky enough for it to be Freedom in Canada (which overlaps with Verizon), and is more likely Bell or Rogers (which overlaps with AT&T).