Have you called Sonos Support directly to discuss it?

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.

Yeah, I was just on with chat. I’d already rebooted, factory reset, etc before I got on with them. All they offered was a 30% discount coupon to the Sonos store if I send the Sub back to them since it’s out of warranty. Pretty soured on the whole thing really.

Interesting, not what I’d expect, certainly. I’ve seen a few threads recently that seemed to suggest removing surround speakers before adding the Sub, then adding the surrounds again. Of course, there is no indication in your post that you even have surround speakers, so that may be a wasted comment. 

I would certainly not factory reset any speakers moving forward, unless instructed to by a Sonos employee. At best, it seems to ‘hide’ an issue temporarily, rather than fixing it, and has the added pain of deleting any stored data from the speakers, which would normally be sent to Sonos when a diagnostic is run, and give them some harder data to look at rather than dealing with anecdotal data. I’d also stick with either the 2.4Ghz channel, or keep it wired, since the surrounds and sub would normally connect via the 5Ghz (hidden) channel, which means never turning off the WiFi radio…but if it isn’t connecting, that may be a moot point. 

I’m curious as to how you connected the Sub via WiFi or LAN, since to my knowledge, it has to be ‘bonded’ to another Sonos speaker, and can’t be set up on its own.