Google OnHub Port Forwarding Not Working

Issue: Port forwarding of external ports to internal devices using the Google OnHub router stopped working

Cause: A device supporting UPnP was recently added to the network. I was able to arrive at this hypothesis because once I swapped in a Linksys router with no static port forwarding configured, the offending device was available and responding externally.

Resolution: Disable UPnP on the offending device.