neighbors' devices jumping into our network???

Since we moved here, we noticed that our internet has been weak & flucuating heavily: consistently dropped calls & bad streaming capabilities; all this is due to 1) our apt complex is in a general cell-signal city dead zone, and 2)our apt walls are thick/concrete with maze-like hallways that interrupt our own wifi  😑 (this on top of rona & wfh-life).

so just today we got the 3-pack deco m5 (mesh wifi system) for our apt. followed the directions and set it up in the early afternoon - works smoothly. over the next 3 hours, we noticed that our neighbors tech (the folks who live diagonally upstairs from us - no walls/ceilings are shared between us) have slowly been connecting to our xfinity wifi.   😕😭😢😭

we had to pause all the devices, ours and the neighbors', via the xfinity app (the one with the home icon). the only thing we unpaused was our deco m5 network ~ which is currently a hidden network, with the WPA2-psk [AES] enabled ~ and our all devices are attached to that network......

my question: how do i get my neighbors off my xfinity (without having to do the unpause thing)?? and avoid this from happening in the future??