This determines how traffic from the server lan is directed to the client lan. If it is just the smartphone that needs to be reached, then I don’t think you need to do anything. But if the smartphone is acting as a hotspot, then it is running its own LAN, so that is the equivalent of the 8.0 or 10.0 you are using. If the smartphone is tethered to a travel router, then I’m not quite sure what you would use-the client router wan or client router lan. But OpenVPN doesn’t care about all the steps along the way; it only needs to know that traffic to that subnet goes down that tunnel.
The next step is whether you allow traffic from 8.0 to go to 10.0 and vice versa, so all three LANs are tied together. I forget that part, to be honest, and at that point you might want to be looking at wireguard–those two peers would communicate directly, and not through the common asus router, which would be encrypting and decrypting traffic twice, and subjecting all traffic to any ISP speed limitations up or down.
Also, looking at the screenshot, I don’t think you necessarily need to allow only the specified allowed clients. If you have a laptop, for example, you might want to connect without worrying about LANs. The GUI is written a little weird–the “allowed clients” table means that if one of the connections made by an allowed client is one of the clients listed in the table, then add those routes.