if your router is properly configured for UPnP, you don't need port forwarding. If you can't set up UPnP properly, you can try port triggering. Port triggering let's multiple devices use the same ports at the same time. it's better than port forwarding which has the limitations listed above.
also, a network is much easier to set up and diagnose if you use manual configurations. give each of them unique static IP addresses that no other devices are using within your network range. you can help ensure this by assigning them DHCP reservations in the router. a little redundant but assign them those IP's also in DHCP reservations makes sure that another device doesn't try to snag the IP when the Xbox is turned off. for the gateway, use your router IP. for dns, try to get the DNS values from your ISP.