With all the above still valid - i.e. gateway working on the version 3 “stack” as well as a couple of applications working all the way to Cayenne, I now find that after a few days (even with no “use”), the gateway goes offline, displaying two solid LEDs and the third slowly blinking (meaning it is “activating”). In that state, the gateway is still connected to the network (I can get on to its webserver).
A powercycle fixes the problem - until the next time…