@jimjoe 

This behavior is quite logical:

You have a double NAT in your network with this configuration.

Once from your ISP modem/router and then again from WR840 (which is in 'wireless router' mode). When in 'wireless router' mode WR840N has its  DHCP ON.

If your WR840N has 'access point' mode use that mode instead and then the DHCP should be OFF since only your ISP modem/router DHCP will be enough.

See here an example of configuring a wireless router in AP mode.

There are two cases:

1. Device has dedicated AP mode in this case you're using second router WAN port.

2. Device doesn't have dedicated AP mode - you were referring this scenario. In this case you're using second router LAN port.

  @jimjoe

Yes, both of your routers DHCP servers are assigning an IP addresses in their LAN subnets.

For example your ISP router's DHCP may assign IP addresses in 192.168.0.1-192.168.0.100 space (just an example).

While your WR840N's DHCP may assigns IP addresses in 192.168.1.1-192.168.1.100 space (just an example).

As you can see there is no conflict between these two subnets cause they differ by its 3-rd octet ('0' or '1' in this example).

That the reason you have an Internet connection and everything works.

While this is working it's better to have all of your devices (whether they are connected to one or the other device) in only one subnet (your ISP router subnet 192.168.0.X for example) - that's the purpose of an AP mode from my previous post - to extend your local network without complicating it.

This way you'll avoid complication in accessing your local devices, gaming issues, complicated network troubleshooting, etc.