just gotten 100mbps unifi recently (on the first floor) and was provided the standard white router. initially it was bad on my desktop (2nd floor) as i was using a 2.4gb receiver. upgraded to one which can receive 5.0 (600mbps) and was enjoying the speed of unifi for the first time (house area has always been streamyx).

then came my my cctv dude who suggested upgrading the whole house to deco as the cctv hub which was installed in the 2nd floor stopped working due to weak signal of the unifi router on the 1st floor. whole house got installed with deco and now my internet is trash again, disconnecting even 1-2min, youtube music stops etc, cant do speed test as it just cuts off.

however on my wife's macbook/phones/laptops, none are facing this issue as what i'm facing on my desktop. can some sifu explain to me the issue? is it my desktop receiver issue? i've tried with 3 wifi receivers on my desktop and non are performing as well as the initiall white unifi router,

in the deco app, try disabling the 'mesh' mode for your desktop, and see if there's any improvement?
I had a similar issue on my laptop some time back.. somehow my laptop was switching between the main deco and the slave every now and then.