One of the reasons I moved to SonosNet was that the speakers were disconnecting from the Amplifi WiFi
since upgrading to the new Sky Router in August, due to the availability of fibre to our house, the Sonos has been unstable and playing up.
I suspect this is down to the Sky router, Sky Q and Sky Mini’s all acting as a Mesh WiFi by default, and possibly generating interference with the Amplifi signal. So at this point you had 2 competing WiFi networks. When you created Sonosnet you created a third one, but, it’s likely the Sky boxes were controlling the network path configuration (STP) not Sonos (which it needs for Sonosnet to function correctly), so even Sonosnet is unreliable.
On the 2.4GHz WiFi spectrum there are really only 3 channels, 1, 6 and 11. With 3 networks, unless you had configured them to each of those 3 different channels, they may well have been interfering with each other. If you have neighbours in close proximity and can see their WiFi networks, with 3 of your own, you are only guaranteeing that you will get interference somewhere.
If the Sky Router is new with the provision of fibre, it might be that you can get away with just the Sky network and lose both the Amplifi and Sonosnet as the Sky boxes will boost the signal around the house.
Unless Sonos Tech support have some way to allow Sonosnet to interact safely with Sky Q, my experience of the two is that it doesn’t work. I ended up buying a Cisco switch so that I could fix STP properly between the two. However, that’s an extreme solution for most.