Not sure.. i tend NOT to use WPS since its very insecure..
General security changes shouldnt have. As you said you factory reset and even exchanged.. its not just a setting.
Something more on the NAT, lower level, is likely causing it.
I run with the CGN3 as a router, just with the wireless off (as i need the ports on it for better network layout), and a 3rd party router running just as an access point... and have not had an issue with things some others have complained with re: the wireless and similar devices.