You would really need to describe your network setup in a little more detail, before suggestions can be made. Here are some things that might help the community to understand your issues.

• Make/model of router?
• Other WiFi access points?
• Wifi channels in use/Channel-width?
• If System is running on S1 or S2 setup?
• If using SonosNet and how many devices wired and where to (distance of wired device from nearest AP)?
• If using SonosNet - if you have chosen to retain the WiFi credentials on your devices?
• Details of primary controller device used?
• Music services used (local and/or remote)
• Average ISP download/upload speed? 
• Anything else you think might be relevant/helpful, like ‘managed’ switch settings etc?

It would also be helpful to know what troubleshooting steps you have taken so far?

Hi

Make/model of router?

BT Smart hub 2

Other WiFi access points?

No

Wifi channels in use/Channel-width?

Smart Channel 36, 5Ghz

If System is running on S1 or S2 setup?

S2

If using SonosNet and how many devices wired and where to (distance of wired device from nearest AP)?

No

If using SonosNet - if you have chosen to retain the WiFi credentials on your devices?

-

Details of primary controller device used?

Samsung Note20 Ultra

Music services used (local and/or remote)

Amazon Unlimited

BBC Sounds

Average ISP download/upload speed? 

133Mbps

27Mbps

Anything else you think might be relevant/helpful,

 like ‘managed’ switch settings etc?

Yes to always use this IP address And Static set for the N20 Ultra

Yes to always use this IP address for 3 of the speakers

The other speaker No to always use this IP address, because it won't let me (it reverts back to No).

Speakers and Ultra all on 5Ghz

I've tried turning speakers off and on at sockets and running through set up/ basic troubleshooting.

Many thanks indeed

Sadly that BT router is quite frowned upon by some users here in the community - you only have to search the forum to see the various issues mentioned. Anyhow…

Maybe try this… cable just one, preferably a left-speaker only, direct to your router and switch the system over to SonosNet. Reboot your speakers and ensure all show WM:0 next to their entries in ‘Settings/System/About My System’ and just see if that quickly resolves your issue.

Just to add, ensure you keep the wired speaker at least one metre, or more, away from your router.

Thanks. I'll need to get a switch to do it. Much appreciated.

Can’t you just move one of the ‘Ones’ near to your router to just try the suggested setup temporarily, to see if it first solves your issue, without going to any expense at this stage?

Thanks, good suggestion. However, I’m a bit wary. I was staying with friends a few weeks ago. They have an Arc, a Sub and two Ones connected as a home theatre. They had put the Ones at either side of the TV. Thinking I was being helpful, I suggested that we move the Ones to the rear of the room for proper surround sound. The left One was plugged into the router by ethernet. It must have been set up as Sonos Net. After powering down the Ones, as soon as we removed the ethernet cable from the Left One, the router disconnected from the internet. There was no way to reconnect it and my friends were without internet for 2 weeks while they waited to be connected to a new ISP because TalkTalk were useless.

Surround speakers can not be used as ‘SonosNet’ generators. They connect to the soundbar on a special hidden 5Ghz channel, which accepts data from the sound bar, but doesn’t ‘send’ data. Not sure why this system ever worked. The only possibility I can think of is because your friend had them as ‘front’ and not ‘surround’ speakers, they were set up as another room, and ‘grouped’ with all the other ’rooms’. 
In that specific case, I’d have wired directly to the sound bar, and let it be the SonosNet generator. All the ‘rooms’ in that system would connect to it. Or there would be two other options…add a BOOST to the equation, and run an Ethernet cable from the router to the BOOST. Or, just let the whole system hang offf of the Wi-Fi itself, following the instructions at the wired and wireless modes FAQ. 

Yes, the Ones were probably set up as front speakers and grouped as you suggest. The left was wired to the router. Apparently Sonos advised to do this when my friend initially phoned for help with set up.

I have no idea at all why the whole internet went down when we unplugged that wire. It seems totally bizarre and almost as if, by sheer coincidence, a fault occurred on the internet line at almost or exactly the same time.

But the fact that it happened, and the consequences, are making me think that my delay issue isn’t so much of a big deal and maybe I’ll just live with it, rather than risking losing my internet connection completely, too.