If you bridge the Motorola, then it is like the Nighthawk is connected directly to the Internet and the Nighthawk does the NAT, security, and DHCP. In this configuration the only thing connected to the Motorola should be the Nighthawk (also, no WIFI from the Motorola).
If you bridge the Nighthawk (page 42 of your Nighthawk manual) then the Motorola does the NAT, security, and DHCP. The Nighthawk acts like a hub and passes packets on the Motorola. With this setup you should be able to use both the Nighthawk and Motorola for WIFI. This is the preferred way of doing this.
If you are lazy or just can't get it to work, then on the Nighthawk, take the line in the WAN port (should be the one going to the Motorola) and plug it into one of the LAN ports. Now the Nighthawk is just a dumb hub (with WIFI ... probably).
Remember one thing ... when you make changes to your network like this, it is quite common that network address, gateway, etc (all that DHCP stuff) needs to be updated. Restarting your device (like the PlayStation) is an easy way to update all that. That or you can do your network setup again.
BTW ... I saw there was a recent firmware update for your Nighthawk ... don't know if that will help you at all.