How can I create two different subnets for 2.4 and 5GHz WiFi, and maintain communication between them?

I have two routers that are interconnected via Cat5 cable: a TP-Link Archer C7 and an old Netgear V7610. I am trying to create two subnets on the same LAN, and maintain communication between all nodes, regardless of which router they're connected to.

The Archer C7 is running OpenWRT and has its 2.4GHz WiFi radio disabled, while the Netgear V7610 is running proprietary firmware from my ISP and has its 5GHz radio disabled. All of this is to workaround the inability of the C7 to use both radios simultaneously. I have been unable to rectify this core issue, since it defies belief by any OpenWRT experts that I've spoken to about it. See this OpenWRT forums thread for more information on that.

I would simply scrap the V7610 and run everything through the C7 on 5GHz if I didn't have some older devices that don't support the newer WiFi standards. So this is the only solution that I can think of, besides running Cat5 all over the house, which would be too costly in terms of both time and money.

I have read through the following resources but am still confused:

Creating a link between two OpenVPN subnets

Communication between two different network id

https://openwrt.org/docs/guide-user/network/routes_configuration

The last one in particular is the first thing that I tried, i.e. setting up a static route from the C7 to the V7610 like so:

192.168.1.0 is my C7's network address, and the V7610 (192.168.2.0) is connected to it through 192.168.1.254. The cable that connects them runs between the WAN port of the V7610 and a LAN port of the C7.

All of this works great except for the issue mentioned in the title. For example, 192.168.1.2 can't communicate with 192.168.2.2. Connectivity between all nodes and the Internet is working fine.

So after that lengthy introduction (sorry!) here are my actual questions:

1. Why does the static route on the C7 not work?
2. Would disabling DHCP on one router or both fix this?
3. How else may I be able to fix this frustrating and baffling issue?

Solution 1:

The cable that connects them runs between the WAN port of the V7610 and a LAN port of the C7.

This can't work, the WAN port is considered an external network, thus the V7610 will (at least by default) use NAT and firewalling to explicitly not allow incoming traffic from it to its iternal network.

I don't know the V7610 specs, but if it has a LAN port (which I hope it has) you should use that to connect the two devices, and then give them appropriate addresses (i.e. in the same IP subnet) on their LAN ports; then you can start thinking about routing.