Since the 6100 is also a router unless it's bridged, that's probably the issue. When you connect directly to the 6100 with a wired connection, do you get a public IP address or a 192.168.x.x address? If it's the latter, that's your problem. Ideally, you'd bridge the 6100 to eliminate it's NAT layer. If that isn't possible, you can configure it to put the secondary router in it's DMZ, which will eliminate port forwarding in the 6100.