Double Nat Issue (LTE modem + OpenWrt router)

[Lynx]

Yes I think I may resort to this now.

In terms of @mk24's well-written approach here:

@mk24

For this example the WAN router's LAN is 192.168.0.1/24 and the OpenWrt router's LAN is 192.168.1.1/24.

In the WAN router first create a DHCP reservation so that the OpenWrt router has a consistent IP address such as 192.168.0.20.
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 (192.168.1.0/24) via 192.168.0.20 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.

Why is it that a static route is needed from upstream router to downstream router but not from downstream router to upstream router?