Thank you for the information! The desktop app for Mac has helped me update the new Sonos One SL. Then after the update, I can connect to that speaker from the mobile IOS app.

Exactly. 

Wouldn’t it be nice if Sonos just posted that on their support page instead of waiting on hold for an hour with customer support?

Thank you. Yes, I know. I called Sonos support and they told me to do just that. And it worked. 

I had this same problem. Try using the Desktop app instead of the mobile app. For some reason, that worked for me and took 5 minutes vs wasting 5 hours.