Seems odd. The first thing I’d stop is the reset of any and all devices, unless instructed by Sonos. As you’ve found, that rarely fixes the issue, and erases any logs that Sonos might be able to have read. 

I’d suggest two things. One you may have done, it isn’t clear from your post the specific order. First, power off (but don’t reset) all Sonos devices, including the Sub. Then, while they’re powered off, reboot that router. Wait two minutes, then plug back in the Sonos devices, and wait two minutes before testing.

If this doesn’t help, I’d 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, they get sensitive about GDPR. They should be able to identify if there’s an issue with the Sub, especially if you wait for it to disconnect before submitting. 

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.

Thanks so much for the timely reply. I’ll give that a try tonight when I get home. 

You say bought used, was that some time back and the Sub has been working properly until now, or just bought and it has never worked right?

If the second you may have bought a defective unit the previous owner was trying to unload and hoping they wouldn't have to refund the purchase.

In either case a call to Sonos may well be in order.

Support may or may not be able to tell you if that Sub has an existing support case. They should be able to reassure it is working properly or is defective so you can work on getting a refund.