Windows seems to recognize the default gateway address, while the gateway itself doesn't, this is getting stranger by the minute. Try reverting the router to it's factory settings, upgrade the firmware again, and change anything that is required to access the internet, but leave out anything unnecessary, like custom DHCP timings, and assigned IP addresses, but make sure to open the ports.
While going nuclear may not be the most efficient of all the solutions, it's probably the most effective over time.