Try unplugging the Arc and Sub from power, and power off the Roam all the way. Once they’re powered off, reboot your router (once, not every time you want to use these). Once the router has rebooted, the plug back in your Sonos Arc and Sub, and turn back on the Roam. 

See if this fixes your problem, which may take a day or two. If it does, I’d heartily recommend that you look in your router’s manual on how to set up reserved IP addresses, if a router has gotten in to this ‘bad’ state once, it could again.

However, if this doesn’t fix the issue, it would help Sonos help you if you were to 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 Sonos system and network.

Thanks @Airgetlam. I tried what you suggested first but it did not solve the problem, so I ended up with your second suggestion: calling Sonos Support. Kudos to their great customer service!! Answered the call in less than a minute, were courteous and clear in their guidance. Recommended I connect Arc to LAN instead of wifi, and that seems to have done the trick