some modems come with two sets of drivers, either supporting PPPoA or PPPoE.
The uk uses PPPoA encapsulation with vpi and vci of 0,38, so that is what the drivers would have installed as (assuming you are using the drivers that came with the device)
Not sure what s.a. uses, but I would be very suprised if it was the same.
Let me know what device it is, and I can help further (i.e. make model, how do you plug it inot your pc...pci or usb etc) (you mention "modem" and "router" in your post, hence my confusion. which one is it? or is it a combined modem/router)