I was having this very same issue, and read this thread with great interest. Also I was having trouble with some devices connecting via WiFi (would connect one day, but be unable the next, even in the same room as the router). This and other issues has convinced me the router firmware is basically just bad. Not horrible, but quite buggy.
I was able to get a stable connection by turning off the WiFi and putting the device into 'Bridge' mode (found under Advanced->WAN settings). This basically turns it into a simple modem, no router capabilities at all. This mean however you need a different device to handle router functions. I had a CradlePoint MBR 95 from my days of using wireless modems (very solid performer). Worked without any configuration. Just ran a network cable from one of the network ports on the Zyxel to the WAN port on the CradlePoint, and poof, internet. It's been stable for the last 24 hours, zero connection drops while gaming or using work VPN (both were a real pain with the drops every 15 mins).
Once you set the Zyxel into Bridge mode, you can't connect to it's GUI pages any longer; it's just a dumb modem now. To get it back to 'normal', you have to do a hard reset using the pinhole button on the back. Both bridging and resetting are detailed on Zyxel's website (via videos hosted on YouTube).
If Zyxel comes out with a firmware update I may give it another try, because I did like some of the monitoring features they offer. But until I feel like rolling the dice again, this setup is working great so far!