Sounds like your speakers may be having trouble with sporadic communication with your router. Rather than just rebooting the router, unplug all Sonos devices first, then reboot the router. Wait two minutes before plugging back in the Sonos devices.
If that doesn’t help, check for wifi interference.
Finally, if there is no help from those options, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here.
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.