You could try temporarily disabling the 5Ghz network.

Unfortunately, this forum is for Sonos users, and not Sonos support directly, as they don’t frequent this area. At best, you may get a response from one of the (very few) moderators who work for Sonos, and a lot of good input from the community, such as the above. 

In some threads, it seems that people have force closed the Sonos controller, then reopened it, and been able to enter their WiFi password and get it accepted. I’d also try, if possible, temporarily connecting the speaker with an Ethernet cable to your router, until it gets set up, and then remove the cable. 

I had to complete set up with Apple cellphone and Ethernet cable plugged into router.  After set-up was complete, I was able to see the System on my Android phone.  I still can’t figure out how to utilize without the Ethernet cable staying plugged in though.  Every time I unplug, I lose the system.

I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, while the cable is plugged in, and call Sonos Support to discuss it.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.