I have a Linksys RVS4000 with the latest v1 firmware (18.104.22.168), an Xbox 360 and Xbox One. Both Xbox systems uses the same ports for Live access, and the Xbox One has a few additional ports.
The port forwarding on the router does not allow overlapping so I cannot configure the same ports for both IPs assigned to the Xbox systems. Both Xbox systems are connected to the same switch which goes to the same VLAN port on the router. Their IPs are 192.168.2.2 and 192.168.2.5 respectively and the network is 192.168.2.0.
In the port forward entry, what I tried was a port forward range of 53-4500 both UDP and TCP, to the network 192.168.2.0. That seemed to work momentarily but on subsequent tests, the Xbox indicates the NAT type is moderate.
Any help with this specific model would be appreciated.