I think I read channel=auto causing issues in the Other thread.
I have no idea what happened, but things are working now.
I've tried following sera's suggestion. I've performed multiple steps that I did not write down, so it might be that some of those steps have made things work (or maybe things are just flaky). However, what puzzles me is that I've tried recreating the settings since before things worked, expecting that they would not work, but they do now...
Here are roughly the steps (that seem relevant) that I followed:
* I used another router that I know to work with OpenWRT, started its 2.4GHz WiFi with the "auto" setting and determined which channel it selected (it selected channel 1)
* I then turned that second router off
* selected channel 1 on the 1900WRT
* things worked (!); thank you sera!
* selected "auto" and things still worked
I will provide any information that is needed to diagnose what the issue was, but I might not be able to reproduce. Anyway, it looks like everything is working now.
Thank you @davidc502!