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. 

Typically, one BOOST is wired to the network and should be spaced at least a couple feet from any other WiFi devices.

If any SONOS device is wired to the network the whole SONOS crew (except MOVE and ROAM) will switch to SonosNet, the SONOS private mesh. BOOST uses the same radio as the players. The audio section has been deleted from BOOST in order to reduce costs. BOOST is typically used in situations where the wired network connection is in a location that you do not want to sit a SONOS player. In difficult situations, such as yours, you can use multiple wired or wireless BOOSTS to improve wireless coverage. A wireless BOOST should be placed about midway between a good coverage area and the problem area. If the BOOST is wireless it is easy to experiment with different locations.

To experiment grab a long extension so you can move it around and not have to put up with the startup delay.

I bought a 100 foot Ethernet cable for the same reason, I can just walk stuff around, sit it down and run my tests.

One minor caveat to buzz’s post...a Sonos device that is “bonded” to a Sonos sound bar connects to a 5Ghz signal created by that sound bar, and not the SonosNet signal. It’s an extremely minor difference, but the sound bar would connect to SonosNet, the surrounds and subs would connect to the sound bar in a normal configuration.