@lazySusan,

If I am correct, the UPnP settings will only affect whether or not the devices connected to the router, will be able to connect. Is there any indication in the logs or otherwise, that the connection is lost between the router and the modem? Many people actually disable the UPnP setting on their routers if they do not use it, so this setting should not result in a complete loss of connection. Also, interestingly enough the downtime for all those disconnects is less than one second. Almost as if it is not properly reassigning IP addresses to devices. Maybe mess with the DHCP lease time settings.

When the issue occurs, take a look at, or take a picture of the LED lights, this is an extremely helpful indicator for discovering where the issue lies.

If you have a hardwired connection, does the internet drop out? (I assume it would)

Do you have any devices set with an IP reservation on the router? This may bypass the UPnP and would indicate an internal router addressing setting is the cause of the issue.

Do the lights on your modem remain consistent when the network drops?

If you do not have many custom IP reservations or settings on the router, I would recommend resetting the router back to factory default; just so that you can make sure that no settings were changed accidentally