I ended up using the browser to go to https://openstore.ubports.com/ and from there was able to search for and download the Open Store app. Installed it, and now it works.

I've just installed UBPorts on my Nexus 5. I opened the Open Store app to see what's available, but all it shows is the message: "fetching package list" and has been for the last 20 minutes. I do have a working wifi connection - verified using the browser.
Is the OpenStore app functional?