Thanks @bmercier
That's very helpful but it introduces a separate point of failure (the external computer that needs to talk to the ISY over the internet every 5 minutes).
I was hoping for something built in to the ISY portal itself that would not require active intervention from an outside computer.
What I was thinking of was that the ISY would periodically ping or otherwise communicate with the portal server (assuming there is such a thing) and if it didn't get a response then it could take an action (in my case run a program that would turn power off to the router and modem and turn it back on). This presumes that the portal server is up with high reliability.
Of course it doesn't have to be the ISY portal that is the outside server performing this function. For example The ISY already set up to periodically get time information from a NTP server or pool. If there were a system variable to indicate whether this update had succeeded, that could be used as the trigger for a program -- that would be all that I would need, thought it would be best if it could retry and only provide the "failure" signal after a couple of retries.