OpenWrt behind a "Connect Box" / Cable Router, enable communication between both subnets

Please forgive me if I ask dumb questions - I simply am who I am.
I have OpenWrt 18.06.1 finally running fine on an Acher C7 V2. After I first I bricked the device, learned about the JTAG option, purchased a PL2023 and followed the forum instructions to bring it back to life. I mention this just to prove that I am persistently willing to learn.
I also have an OpenVPN Client (configured for IPV4 only) connecting to a commercial VPN provider running fine, which gives me the choice between my ISP and VPN for access to the internet, depending on to which router the connection goes.
My setup looks like this:

Internet <--> Connect Box [] <--> WAN Port Archer C7 []
VPN Internet <-------------------------------OpenVPN------------------------------------^

I would like to interconnect all devices on both subnets in both directions. Upwards works fine obviously (from 192.168.1 to 192168.0) but downwards does not work, even if I bind a valid second 192.168.1 IP address a device connected to the 192.168.0 subnet. I understand that OpenWRT firewalls its clients (connected to the Archer) from incoming connections through the WAN port - supposedly internet connections. But in my case it is not the internet but only my harmless 192.168.0 subnet.
Be aware that the Connect Box can basically not be configured at all, except for the the MTU size. Please give me a hint how to enable communication between my devices on both subnets.

It will not work without a route to lower level network on your upper level router/gateway.

The only alternative is setting up lower level router as switch.

Thanks vgaetera. I was hoping I could fool that unconfigurable connect box with something like using the same class c network on both subnets, limiting the DHCP scope on the upper device to the upper half and on the lower device for the lower half of IP addresses. I will leave it as it is as I can easily connect to of the devices if needed.

One way to allow access to is to setup a vpn server on openwrt and vpn clients on

Maybe there is a better solution. What is the purpose of