high-end router or Sonos boost?

Are you running on wifi or SonosNet? The latter is a mesh network, AIUI, and may give you better performance. 

Hi.  If you are using a wired Boost then you are using SonosNet, and the Sonos speakers are almost certainly not using the router wifi (except for Move and Roam).  However, I am not convinced that this is what is causing your dropouts.  Not convinced it is the brick chimney either.  It might easily be WiFi interference that is causing the problem.

Provided the wired Boost is at least a metre from the router it should not present a problem.

In About My System do your Sonos speakers have WM:0 or WM:1 next to them?

What Sonos products do you have?

What 2.4GHz channel are you using on Sonos?  What about router?

Is there an option to NOT use SonosNet? How would I turn that off?

I have tried a mesh network before to try to overcome my issue but it was actually worse. The layout of my home is C-shaped with the chimney in the middle. The locations of my power outlets necessitated that the signal must still pass through the brick walls of the chimney, and the mesh pods were actually worse at that than my bog-standard ISP router.  Unfortunately there's no way around the chimney aside from running ethernet cables though the flat, which kind of negates the point of having Sonos speakers. 

Thanks for the comments. I will have a look at the settings again tonight. What is WM:0 and WM:1?

It's definitely the chimney though. I have traced the signal throughout the home. I should probably add that its an old building and the chimney is more like a smoke-stack. The size is comparable to a elevator shaft. 

It's interesting what you said about the Roam and Move. I have 2 Roams- do they not communicate with Sonos Boosts? Because certainly the Roams are the speakers furthest away from the source.

WM:0 means connected  by Ethernet or SonosNet.  WM:1 means connected to WiFi.

The Roam can connect to WiFi only.

Interesting.

I unplugged my Boost and signal seems better. (Edit- WAY BETTER WITHOUT BOOSTS) Perhaps it's time to sell my Boosts?

My Port is WM0 (its on an ethernet cable).  My Roams and Symphonisk bookshelf are on WM1. My Arc and Surrounds are on WM2.

What is WM2? Is that HDMI?

WM:2 is the dedicated 5 Ghz connection between the soundbar or Amp and the surrounds.

If your Port is connected by cable, it starts a Sonosnet (like the Boost does). All your you orther devices should revert to WM:0.

Boost and not Bridge? The Bridge is very old WiFi technology.

I find restricting my WiFi system to modes that Sonos can work with reduced the capabilities when connecting with more modern devices, didn’t care for that. With a directly wired to Ethernet SonosNet v2 capable Sonos speaker (not a sub or surround though) or a Boost I get full performance from my Sonos and from my newer tech WiFi gear.

With two Boosts and multiple speakers you hopefully should be able to come up with a setup that works around your obstructions. One consideration is spacing, too close is bad, a meter away from any RF generating device (with or without a radio) is usually a good idea.

Have you looked at your Network Matrix to see if it holds any clues?

http://your-speaker-ip:1400/support/review  Not available from a Boost/Bridge.

Day 2 without the Sonos Boosts and the signal to speaker connectivity is much better. 

I am beginning to come to the conclusion that the Sonos Boost just isn't strong enough to penetrate my concrete floors and brick walls. The Netgear router is proving the superior option. 

Perhaps adding a 3rd Boost to my system might help but I don't see the point now the router seems to be working adequately.

Which leads to my next question: does Sonos buy back used Boosts?

If the device is still under their ‘trial’ period, you can return it. They do not ‘buy back’ any devices.