Coupe of thoughts. 1) The netgear firmware may have updated, so that may be a change that you’re not really cognizant of. 2) Sonos has always said that they’re not compatible with network extenders….simply put, it seems that some work, others may not. Given the status of 1), things might have changed that caused your Sonos situation to change. 

I’d try, at least temporarily if necessary, wiring your Amp directly to the base Netgear router. If that “fixes” stuff, then I’d consider one of several options,  a Sonos BOOST wired to that router, or a direct wire to another Sonos speaker to generate SonosNet, or simply inputting your network information into the Sonos controller, and after that, removing any ethernet wiring at all. 

I’ve had cases where Netgear have updated my router without my knowledge. And Western Digital has updated my NAS. And Vizio has updated my TV set. All without any notification to me, and in each case, some “minor” change to “help” has caused failure in various parts of my Sonos network, requiring me to go into the individual settings area and make specific changes, often reverting what the manufacturer “thought” was “better” (ever notice I use a lot of quotes? )

Thanks for your thoughts Airgetlam.  A few things on your comments.

I know with certainty that my router and extender have not be updated because I turned off automatic updates when I set them up many years ago.

I now know (from Sonos and this forum) that an extender is not supported.  However, it worked just fine for 2.5 years.  So I come back to “what changed”.  Only thing that may have changed is the Amp and/or Sub was updated, and they busted something.

I cannot physically wire the Amp to the router given locations in the house (different floors, don’t want to start drilling).  So I’ll see how things work with the extender turned off and the Sub closer to the Amp.

But is that a thing?  That is, does the Sub actually “talk” directly to the Amp and they have to be within some range of each other?  I thought it all happened over Wi-Fi.  (I’m a software guy, not hardware, so what do I know.)

I don’t know “what changed” in your system. I’ve got zero data other than what you’ve reported in your posts, and my own previous experience with my own Sonos system. Nor am I a programmer at Sonos, to be aware of what may or may not have changed in their software, but it seems to me that unless there was some reason for them to be messing in that area  of the code base, it’s unlikely to be some odd random change. Perhaps we’ll never know, Sonos doesn’t share their code base, nor do they provide specific data as to what lines have been changed in their software. 

Yes, the Sonos Sub connects (normally) wirelessly to the Sonos Amp on a 5 Ghz signal (hidden, not your normal wifi) , and should be within some range that 5Ghz can traverse. There are cases where the data can take a much more convoluted path, to my knowledge, to travel across an ethernet cable, but it’s not the way the system was ever designed to operate, and I’ve personally always considered it to be suspect. May work in some cases, and may not in others.