Rather than go through the various config changes I've tried, I'd rather start with the desired configuration because I'm clearly lacking some basic conceptual understandings:
I have OpenWRT 19.07.4 installed on two devices.
The Linksys device is connected to the cable modem through the ethernet WAN port.
The Buffalo device is connected to the Linksys device as a WiFi client.
My preference is for the Buffalo device to manage its network as a router rather than simply bridging the networks.
Using the LUCI GUI, what steps should I be taking?
When I want to configure port forwarding from behind the Buffalo device, again using the LUCI GUI, what steps should I be taking?
Note that the client I am using to test port forwarding is Transmission from Linux Mint (and, no, this not for illegal file sharing).
I realize this all is likely very basic setup steps, but the last hour or so I have only discovered posts that involve more complex configurations than I am seeking or involve configuring non-OpenWRT hardware (i.e., I haven 't discovered effective search terms or don't understand what I'm reading).