Preventing double NAT

For this example the WAN router's LAN is and the OpenWrt router's LAN is

In the WAN router first create a DHCP reservation so that the OpenWrt router has a consistent IP address such as
If you chose an IP different from the existing leased IP, restart the OpenWrt router or at least restart the network so it picks up the new IP.
Then add a route for the second LAN ( via which is the reserved IP of the OpenWrt router on the first router's LAN.
These changes should not otherwise affect the operation of the WAN router at all even if the OpenWrt router is not connected.
Then go to the OpenWrt router firewall setting and uncheck "masquerade" on the WAN and restart the firewall. No more double NAT.