I’ve recently developed the same problem. Our local mast seemed to have an issue or maintenance over a weekend a couple of weeks ago and ever since I have the problem which is repeatable on both my NR5103E and also a Zyxel FWA710. 

in my case if I run a speed test via Ethernet (I don’t use WiFi to the router), the upload test causes the connection to drop. All devices remain connected but even the Zyxel itself knows it’s lost internet connectivity. A reboot fixes it until next time.