Because unless you have both systems port forwarded the Xbox changes the IP address when it restarts.
The only way I've been able to successfully connect without port forwarding is to reset both consoles then resetting my router with the factory reset button. You will need a pen or something similar to be able to push this button.
Be warned however that this wipes the router, including any saved settings and passwords.
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You can only port forward to one IP at a time. Additionally, if you set a static IP to your xbox one it will not change. UPnP is supposed to handle the traffic correctly but after doing some research I am finding that some routers/firmware handle UPnP better than others. It would be great if Microsoft put out a list of approved/compatible routers for using multiple Xboxes.