I suggest that you contact Sonos tech support.  I had no end of issues with Google mesh wi-fi and got rid of it in the end, so your eero mesh may be causing the same issues.

Thanks for the quick comment!

I am really wondering why this speaker (and others) used to work on the same wifi network, then stopped a few months ago. There is something seriously wrong with these temperamental devices.

Sonos speakers are not temperamental. I have had dozens down the years and all have been rock solid. Google mesh has a few hand grenades to chuck under Sonos, and the whole setup needs careful consideration.

I think you should follow ​@UKMedia 's advice to call Sonos Support..

FWIW, if you are still using your ISP router and haven't put it into modem only mode then that might explain part of the.problem. 

Thanks for the quick reply. I am glad to hear your systems are behaving and you do not suffer from any issues so many others are enjoying with our devices! (Anecdotally 5 in my case, over three houses, all but one with intermittent connectivity issues). 

Sonos support has been very friendly, but not entirely effective, so far, but I will definitely try them again. 
 

I would be curious to hear why having another wifi network on the router (which I understand is what you imply) would cause problems, as long as the devices are all connected to the same mesh wifi? It is not uncommon for people to run different wireless networks for different purposes. 

All of the SONOS units and their controllers must be on the same subnet. I have two WiFi networks, but they are on the same subnet. There is only one router. The SONOS units are free to use either network. If one network goes down for maintenance, the SONOS players will flip to the other without a fuss.

Thanks @buzz for the info. This is also how I understood it. 
 

I thought I would provide an update here: after a (long) call with Sonos support, it turns out they think the wifi card in the unit is dead. So the options are to use it wired, or get a 30% rebate for an out-of-warranty replacement, which I understand requires shipping the product back. (Which, while a nice gesture, does not seem a much better deal than the regular 15% upgrade rebate with which we can keep the unit). 

Relatively rare case, but it is an electronic device, and no matter who makes them, there can be issues. Sorry you got hit.