Well, all you should have to do is a "copy tftp start". Then it will ask for the IP for the TFTP server. It really doesn't matter what connections it travels over, just as long as there is a route to the network of the TFTP server. Let us know the IP of the TFTP server, and then some config's from your routers would be nice.... As long as you can ping the TFTP server, then you should be able to upload/download to it.
burke