XB7 Gateway shows online devices as offline

I had a lot of trouble with my prior gateway, the XB3.  Devices seemed to connect at random.  I think there was an undocumented limit to the number of devices I could connect through the gateway.  I have eight servers that connect through wired connections.  My wi-fi devices would wrestle for the remaining slots.  They would sometimes connect, sometimes not.   Windows would occasionally tell me that my printer was offline, but it wasn't.  The gateway was just refusing to connect.

The XB7 is much more consistent.  My wireless printer now works reliably.  I have astonishing download speed (942MB/s).  But why does the router report devices as online or offline seemingly at random?  Everything works fine but I feel like I have no control.  I thought the Xfinity app might be better.  But it does the same thing.  Be clear that I'm complaining about the software that is supposed to control the Gateway.  My devices work fine.  So, can somebody tell me how the interface to the router, either over the web or the cell phone app, is supposed to work?

I know what you're thinking, your devices work fine.  Why are you poking the bear?  But a flaky interface makes it impossible for me to feel that I can control the bear.  If I wake up paranoid tomorrow and think somebody has hacked my system, how do I identify my own devices and make sure something from the outside hasn't connected to my router? 

Something specific has me puzzled.  There is a checkbox on the web interface under Devices.  Prefer Private Connection.  What does that mean?  I tried checking and unchecking it.  It seems to do nothing, possibly because my whole network is private.  But why is it there at all?