I would disable the 2.4Ghz on the ISP modem as it is too close to the 645s. Enable 5Ghz though since the 645 doesn't have 5Ghz support.
I think I need to update that sticky. I think I was using a picture repo that became a pay site so I removed the links.
See if you can follow the information and input the reserved IP address for the xbox from and to sections for local IP address range, remote should be 0.0.0.1 thru 255.255.255.254 and then set protocol to both and ANY for port if I can remember correctly. Save settings then reboot. While the router reboots, do a factory reset under the networking settings this is for a 360, turn off after the reset, then back on. Then check NAT.