I’d go into your router’s settings, and set up reserved IP addresses for all of your Sonos, including the Sub, which gets a proxied IP address through the speaker it is bonded to. 

However, if that doesn’t resolve the issue, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, 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 phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Thanks for yr suggestion, will try