Erm, okay that’s not looking promising. It’s starting to look like Router A doesn’t like this setup at all. It’s not routing B’s traffic, possibly because it’s not the subnet it expects to serve. Ugh. Check all the options you can in Router A if you can find something that will allow it to work.
You can fairly easily test that by enabling masquerading on B. It’ll break most of what we just set up but it’ll confirm that.
We still have some options on the OpenWRT side to make it masquerade only public traffic but now I’m wondering if A will even let you port forward to something on B. I would try that now and see if it works.
Is A able to ping B and devices on B, or only on A? A itself has a route for B’s subnet right?