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... What's your question? How is this physically setup? A diagram would help a lot as would more information.
The proper way to do this is to change the settings on the ISP's router to convert it to a bridge and plug your gaming rig into your router. Then you only have one device performing routing.
A better setup would be to take the WDR4300 and set it up as an access point instead of router. Disable DHCP on the WDR, and take the LAN wire and plug it into one of the LAN ports on the WDR instead of the WAN port. You would have to setup the WDR with a Static IP that is within the range of the ISP Router, and you should be good to go.
If the base firmware of the WDR doesn't allow for Access Point setups, then consider changing it to something like DD-WRT. There are plenty of Google'able places to learn how to install and configure DDWRT as an Access Point.
A better setup would be to take the WDR4300 and set it up as an access point instead of router. Disable DHCP on the WDR, and take the LAN wire and plug it into one of the LAN ports on the WDR instead of the WAN port. You would have to setup the WDR with a Static IP that is within the range of the ISP Router, and you should be good to go.
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