Modem is hard wired for 192.168.0.1 and it can't be changed or shared Set the routers LAN IP to 192.168.1.1 Enable DHCP in the router Disable QoS in the router. Monitor your usage very carefully with the Xbox connected.
I suspect the same thing too, especially if the games on the xbox have not all been recently updated, they will update if he has not played the game since the last update was put out. The NAT error you are getting should not really affect anything with the Xbox unless you try to play something online (keep in mind not all games will work online on the Xbox) and for the best connection, we recommend plugging the Xbox right into the router instead of directly into the modem.
As for the phone and laptop - what models are they and how recently have they had their operating systems updated? iPhone OS updates can be really big and if you're running Windows 10 on your laptop, we're looking at needing to help you make some tweaks to your system.