maybe you can try and nail down the issue by setting up the guest network and turn off 5ghz for that network. see if that connects.

I have my NAS setup like this and it's fine...I actually have the 2 deco pucks on second switch attached to the first. NAS is on the first switch like your diagram.

Interesting...I've had similar dropout issues with google devices, in particular google homes and hub. I had this with the Orbi mesh router as well before I switched to the Deco M5. So even though...

Yes, you do, and set the deco to run in AP mode.

Whilst travelling recently the deco M5 app notified of a firmware upgrade. Because I was away on business I ignored the update,. Now back home the deco app wrongly says there is no update available...