I haven't experienced the internet drops - I run my own PBX and it is really sensitive to drops. I have nothing in the PBX logs that would suggest that happening, although I did see this on my old Netgear router quite often, the 66u runs pretty clean.
I'm seeing the same thing on my AC-66U B1.
The web interface does not properly display and count attached devices, my Android App only shows WiFi attached devices. Seems odd that Ethernet clients don't show at all using the web or the app.
Can you see Ethernet attached devices?