It is like @eibgrad says no network discovery between subnets.
You can reach clients from upstream to downstream but only when using the IP address.
If you want that I have attached instructions how to do that.
I do not have instructions to use a VPN on a WAP but in essence it is as simple as setting the gateway on the clients to point to the WAP as @eibgrad explained.
Client side, setup local LAN clients to use the WAP
To see how you can setup clients to use a WAP
I had to look up Wireguard as I didn't know what that was. So it is a VPN company. I have IPVanish that I am paying for already.
"setting the gateway on the clients to point to the WAP". Is the "client" the dd-wrt router? and how do I point it to a WAP? And if I manage to understand and do this then I can share files and printers for all the computers in the house while still being able to use IPVanish Open VPN when I need it?
I can do most of this stuff with explicit step by step instructions but a generalized statement is beyond me without step by step instructions. I am self taught on all this stuff, spending hours reading how to documents to soak it all in. If they would have had computers in the classrooms back when I was in high school I would have loved to have made a career of this type of stuff but as it I'm pushing retirement and feel greatful that I manage to do what I've done so far.
I'm a newbie too and feel your pain. Sorry I can't help solve your current challenge. However, I want to clarify something. I think the WireGuard people are referring to is not a VPN service.
WireGuard is VPN protocol like OpenVPN. If you activate WireGuard (or OpenVPN) in your dd-wrt router then you can configure it so that you can access your home local network from anywhere in the world using your laptop or smartphone.