Samsung Galaxy Mega 6.3 hits FCC
Early reports said the Mega 6.3 would have both LTE and HSPA+ radios, but the one that has hit the FCC doesn't. This is model I9200, it only has HSPA+ listed, and the bands listed only match AT&T. It would work on T-Mobile in areas that have the 1900MHz spectrum turned on, but the device doesn't have T-Mobile's 1700 or 2100MHz bands.
It's possible this is the AT&T variant, but it seems more likely that this is a variant for AT&T MVNOs which use the carrier's HSPA+ network, but not the LTE network.