@Kevin_Z
Thanks for you reply. It took me a while to sit down and test some things.
What is the current network diagram? How do you connect these devices together?
I am not sure how to draw a network diagram. I can describe how I set up the Decos though. We have one Deco (A) at the front of the house (I might take this more to the middle/back of the house). Deco A is connected to the router where the internet cable enters the house. Connected to Deco A are usually a Sonos, an Apple TV (not streaming at the same time), and up to 2 phones and laptops.
A second Deco (B) is in a building in our backyard, approx.. 10-15 meters from the other Deco. We use the building as a home office where up to two laptops are connected. When we set it up, the connection seemed to work and we have not experienced issues while working there.
The only thing I might do is move my Deco a bit closer to the back of the house. But I don’t see how this might improve the Sonos' connection.
Who is your internet service provider and what is the bandwidth you paid for your ISP?
Our internet service provider is Ziggo. The bandwidth is 50Mbps down and 5Mbps up. So this matches quite well with the measured connection speed.
When the sonos or PC stops working, please show us a screenshot of the speed test page on the Deco app.
I don’t observe issues when checking the internet speed while the Sonos has issue with the playback, see the result (in Dutch) attached. I did not experience issue with my laptop anymore, so haven’t been able to capture this. My guess is that the connection drop is too brief?

In the meantime, with computer connected to the Deco, please test the actual speed you can get and show us a picture also.
All the above tests were when my computer was online too.
What is more, it is suggested to disable fast roaming and setup application QoS as streaming to give it a go.
Fast roaming was already disabled. I set up the QoS to streaming and it seems to improve it a bit, but there were still issues when streaming radio.
Still the same, setup QoS as standard and clear all the high priority rules.
This was the setup I had before. But am going to continue with QoS set to streaming for now.
Kind regards,
Bart
P.S. I also got in touch with Sonos support to see if they know what might be going on, since I mostly/only experience issues with the Sonos speaker.