What did you connect the cable to, if you’re mooching off of someone else’s wifi?

It’s a mirror router, I think, from another house - landlords. I can connect a cable, but I can’t get into the router and change settings. 
it’s a temporary solution until the service provider can connect up my own.

could be that some services or ports, like for ftp, are blocked by the router’s firewall settings

Might be, but how can it block one product but not others? Is the One SL so different from a Play Five? 

Shouldn’t be. It should be the same location for the update software. 

Given this odd situation, I’d try something that isn’t normal at all. Given that you have other Sonos devices, this shouldn’t be an issue, but for anyone else who only have one speaker, don’t do this!

Wire the Sonos Five to the router you have access to, just as a precaution. Then factory reset only that errant Sonos One SL, and re-add as if it were new to your S2 system. As part of the ‘add’ process, it should properly update the S2 OS, using the SonosNet connection from your Sonos Five. Once it is updated, I ‘think’ you should be fine to remove the Ethernet cable, and run your full system.

Just as a side comment, in this situation where I didn’t ‘control’ the WiFi situation, I’d be using a travel router to connect to that other WiFi, and be creating my own WiFi network that I actually had more control over. More protection that way. 

I have no idea why, but that worked. One SL is now up and running along with the other speakers. 
 

As a side effect I can’t connect with any of the speakers through the Spotify app, but here’s the kicker - I can play through Spotify if I go via the Sonos app. Weird. I’ll troubleshoot a bit on my own and make a new question if it persists. 
Thanks a lot for the help! 
 

To the sidecomment; I actually have my own router. Why I didn’t use that is a good question I don’t have a good answer for. Thanks for looking out!